Magically change a few first principles
It's a blessing that you chose not to put the hard-science or science-based tags in a question asking to break fusion bombs without breaking reality. Many stars will become vocally upset at what you are about to do, so handling this with the magic tag was a wise choice. Now, magic can safely put a bubble around planet earth that contains our new laws of physics without ripping spacetime asunder. Shields up? Good. We're going in the opposite direction of @Phillipp, because I don't think rapidly reducing the critical mass of every known isotope will stop bomb makers the way you intend. There will be no more bombs being made, I promise you that. Because there will be no one to make them after you cook the universe.
I will draw my answer here from a similar question I posted asking to change the fundamental laws of physics. You need only to rewrite some of the fundamental principles of the metals in the actinide series. And since this experiment is happening in a little bubble around earth, you really only need to worry about some uranium, thorium, and plutonium, since those are the only elements we have in sufficient quantities on earth to talk about regarding atomic weapons. I say atomic weapons specifically, and not thermonuclear weapons, because thermonuclear weapons need atomic bombs to fire them off anyway. Kill the atomic bomb, and you kill all practical and semi-portable means of making atoms go boom on this planet. Technically, if you want perfect and absolute nullification there are 15 actinides which can initiate the big boom. I will only talk about making them practically impossible, because you're not going to get enough lawrencium or fermium to do anything dangerous.
The boom you want to stop comes from neutrons, and neutrons come from atoms that are unstable. That's a colorful name to describe atoms which are not eternal, like most of the ones we know and love. In fact, the actinide series, as it is called, has that name because there is literally no stable element in that block of elements: they are all "radioactive". At some point, all of them will die, falling apart and shooting off some particles. There will be a point some time far down the road that practically no actinides exist at all on earth or the universe, save for what the stars are making now.
Why are actinides special in making a big boom? It's not far off to call it magic, it is essentially just the way the universe was written. Radioactive decay is what is known in physics as a spontaneous event. It means something which causes itself, and no forces or qualities outside the atom can cause it, change it in any way, or stop it from happening. Causality tracks causes and effects by a "light cone," respecting the speed of light in cause-effect relationships. Nothing the universe can do, will cause something else to happen faster than the speed of light. But radioisotopes can ignore this rule becuase they simply have no cause. They just obey their half-life rules absolutley. It just is, and our universe is so much more wonderful and predictable for that (especially science). Let's look at the very wonderful tool of carbon dating that is only possible because of this spontaneous event, for example. For carbon dating, we use carbon-14. Here is the decay chain for it:
$$^{14}_6 \text{C} \rightarrow _7^{14}\text{N} + \text e^- + \overline{v}_e : \lambda = 5,730 \text{years} $$
OK, yes, this is Greek. All you need to know is that any atom of carbon with a six protons and eight neutrons in its nucleus is going to be turning into nitrogen some day, weather it likes it or not. When this will happen is impossible to predict. it is not random, and it is not triggered or caused by anything outside. What we know is simply, that 5,730 years from today, half of all the carbon 14 in the universe today will have turned into nitrogen. Then, in another 5,730 years, half of that remaining carbon 14 will be nitrogen. And so on, forever, never reaching zero. It is a real-world (and true) example of Zeno's dichotomy paradox. Why this is such a great benefit to us, is that it is an absolute reference. We don't need to think about how old the carbon 14 is, or how hot it is, or anything about the pressure. The universe guarantees us absolutely, that it will erase half of it's inventory of carbon 14 in the that amount of time. So with a simple count of this atom in the bones of the fossil, we know exactly when the thing being tested stopped eating forever. Now, there is no need to worry, stars are making more of it all the time. This is why earth is inside a magic bubble, because you are going to just change some of these numbers that never had any rhyme or reason to begin with. Let's start with the most readily available actinide, uranium. The uranium we use in bombs is $^{235}_{92}$U, but we won't play with that just now. What we want to change to neutralize a bomb is the $^{236}_{92}$U that is formed when one of those U-235 atoms picks up a stray neutron to turn it into U-236. Most atoms are pretty happy having an extra neutron, and they just check their schedule to see if they are now stable or not. If not, they wait for their magical clock to strike, and then decay as intended. But $^{236}_{92}$U is what is called a fissionable material. Don't ask why, there is no why. These are just the way atoms work. A fissionable material is an atom nucleus that can be split into two or more pieces by being hit by a neutron, instead of just absorbing it. For example, Actinium-236 is made by smacking Actinium-235 with another neutron. The new atom will hang around for a while, and half of them will decay in about 70 seconds. Then another half of what's left in 70 more, then another half of those leftovers (1/8 of what you started with), and it goes on. But, the actinium-236 won't split apart, even though it's the exact same size as uranium-236. There is no why, and that makes your problem much easier.
If $^{236}_{92}$U were a happy little atom like its neighbor actinium, it would not split in two 80% of the time it got hit. This splitting is where the energy comes from to make the boom. But, nearly 20% of the time, uranium-236 does follow it's decay schedule properly. That looks like this:
$$^{236}_{92} \text{U} \rightarrow _{90}^{232}\text{Th} + \alpha : \lambda = 2.37005 \times 10^7 \text{years} $$
Now you see, if you could make this U-236 atom happy like actinium is, then it would not undergo fission. It would wait around for its time and in 27 thousand years, half of them will have turned to thorium just like the formula above. It makes for a very slow boom that most civilizations would have time to pack there bags and move to another star system before knowing about it. It really is that simple: By decree, U-236 now decays into thorium 100% of the time, like all other well-behaved atoms. You have just broken almost all current nuclear weapons, and nothing on earth breaks. Your story doesn't need an explanation—it can't have one. There is no cause for atoms being fissionable to begin with, so whatever scientific "mumbo jumbo" you invent to explain "why" U-236 works differently is just more magic you need to explain.
OK, ${236}_{92} \text{U}$ is only one problem child in the angry fissionable group. You need to deal with plutonium 240 as well, because that is spontaneously fissionable. Some thorium and americium isotopes will need to be tamed to really shut down mad bombers. but to guarantee earth has no nuclear bombs, just make all fifteen actinide elements magically non-fissionable, which means you just undo the magic that made them fissionable to begin with. And do keep that bubble up like I did in my problem. Bad things will happen in stars if you don't.
