By definition a reactionless drive has no fuel and there is no cost to travel, that's why they're on the "friends don't let friends have" list for writing sci-fi.
Frame challenge: there isn't anything that any military or police can do to prevent abuses of a reactionless drive that doesn't require fuel and produces nothing else but unlimited amount of force**
** it needs to be force, otherwise the passengers would not be in danger of high acceleration
Some examples of destructive actions neither military or the police can prevent or have any control, no matter how they will try to give themselves an advantage:
MAD? Forget Mutual, it's only Assured Destruction. You won't have time to react when some of those drop from orbit at huge acceleration over your major HQ. Guess what? Even your run-of-the-mill obscure Apocalypse sect can trigger it. With the mush of their martyrs spread thin on the back wall of the hull after they engaged the throttle on a precomputed attack vector.
If the fuel is too expensive for your Apocalypse sect, no worries, some petty dictator will find the money (or one of his decrepit generals that just realized he wasted his life for nothing).
Or an African chieftain, with a diamond mine on his controlled territory, who is just annoyed of those peace keeping operations, who are they to tell him his abuses aren't OK?
You don't yet know what road rage is. Never before you tried it at a non-trivial fraction of the speed of light, but now you can. Guaranteed unique lifetime experience, kids-friendly, take them with you.
Remember those flashmobing that were popular enough before covid pandemic? Imagine one with all the comancheros bikies from all over the world, one that you can't restrict on a 2D space of the planet's surface. How much police do you say could mobilize?
Did you say you can deliver custom size hulls? May I have about a thousand of them, in 4 inches/10cm form factor, with an RasPi connected to the controls? Oh, none at all, no RC option, thanks. Just an USB port, please, I want to pla... errr, conduct my research into AI. Pure defensive purposes, I want to see how long will take the police to disable a whole swarm AI, driven by simple individual rules that keep track of their neighbors, when you give the entire swarm a purpose. Sure, just academic work.
That acned hacker in his mother basement? He's bored after he hacked FBI's email server yesterday (and his heart-crush is still not impressed). So how about hacking today through the police/military system that keep the technology in check for civilians. Just for the lulz, see?
Maybe it's time to realize the consequence of:
- any powerful tool is a powerful weapon too
- the moment a technology is available to the masses, you can no longer exercise absolute control of it. Look, Apple is controlling the iPhone ecosystem in almost every aspect of it: you can still jailbreak their devices;
- access to space involves handling levels of energy far beyond what a lot of the humans (of today) can handle responsibly