This question is very close to a hypothetical “What would happen if…”. What type of world are you trying to create? Are you telling a story? Is this particular magic needed to tell that story or make it more interesting? If it is not important, but an aspect of a large system which must be included to make logical sense, you can just sort of ignore it and state that mages do not use it due to the harm it causes. Manipulating radioactive decay is dangerous and would likely cause illness and death. If the magic is needed for your world to function properly, you should have an idea as to how it needs to be used to move the story along. Just because it can be used in a specific manner does not mean it will be. Alcohol can be burned to produce light, but people still tended to make it to drink instead of burn. People need to see its utilization as practical and useful.
The fact that radioactive decay can be halted is an amazing ability. It means that historically dangerous materials can be handled safely. Can these materials be “locked” into a specific state (i.e. continuously emitting radiation at a higher rate without the constant attention of a mage)? Certain substances would still be impossible to hold in a non-decay state due to the speed at which the element decays. When you are dealing with a half-life of yoctoseconds there is not enough time to capture a stable element, nor would other elements of the same type be able to provide any sort of buffer due to the rapid nature of their decay. Although… that could be a reason why in your world that certain elements decay so quickly. Their stabilized forms are so useful, and used so heavily, that the rest of the universe believes they decay extremely rapidly.
That being said, we can look at the history of mankind to determine likely avenues of utilization. Aside from the detrimental health impact of ionizing radiation, the primary outcome of decay is heat. In a medieval/fantasy type setting, heat is one of the most important elements in society. Heat was used to warm homes, cook food, smelt metals, and for warfare. Lead metal has been used for thousands of years and makes excellent radiation shielding. I could see it being combined with radioactive materials to make heating bricks for warming homes and ceramic walled ovens with iron grill plates for food preparation. There are limitations due to the low melting point of lead and its toxicity, but there is another element which is great for shielding and perfectly safe… gold.
Gold melts at 1.06K C, which means heat from it could be used to smelt tin, zinc, lead, aluminum, and even silver. Bath/steam houses could utilize gold radio-heat bricks for water heating. A small gold radio-heat sphere could potentially be added to a cooking pot and used as a heat source instead of burning wood. Cooking without fire would be revolutionary. Not only did gathering firewood take time and energy, but trees needed to be seasoned prior to use, which takes time. Enough of a stockpile needed to be gathered to last through wintertime to prevent needing to gather firewood in the cold. Smoke has a detrimental impact on the local environment. Creosote buildup in chimneys can cause fires which devastated entire towns. Medieval cities often relied on shipments of wood from distant locations. Crops and foodstuffs were grown near cities when possible. This meant that the land was cleared of trees to make way for fields. Cities built near rivers could sometimes rely on upriver logging efforts to supply their wood demand. Removing firewood as a main fuel would be a game changer.
Depending on your world’s metallurgic technology level, Osmium would make for excellent shielding and has a melting point of 2.92K C, which would allow for the smelting of iron, steel, copper, nickel, titanium, gold, platinum, chromium, and cobalt.
The use of this magic in warfare depends heavily on the range. Can a mage effect radioactive material a kilometer away or merely arms-length? The ability to control radioactive decay would likely result in the study and understanding of how quickly the various radioactive substances heat up, and for how long they can sustain various levels of heat. Knowing that, and the average travel time of a catapult/trebuchet projectile, it could be used to create radioactive firebombs. Radioactively “salting” of fields and other important locations could render them effectively dead until a mage comes in to neutralize the decay and the radiation decreases to a safe level. I could even see handheld (or platform-based) radiation projectors being used by mages to irradiate enemy soldiers. Siege warfare would be severely affected by such capabilities. Arrows with small amounts of radioactive material embedded could become extra dangerous, starting fires, irradiating impaled persons, or even irradiating a location for area denial. Adding lead to armor would likely be a necessity.
If your mages can make magics which pale in comparison to the utilization of radioactive decay, it could be seen as a “poor man’s” magic. People who cannot afford the safe alternative would turn to the cheaper option.