I use a few different tricks, depending on how much I want to destroy in the process. If I'm lazy, I just cut them (RC18T tires are a little bigger than the Mini tire, so there's extra tire there to use if you cut it. Not recommended as re-gluing becomes a pain if you're cutting off the rim it's going back on to)

If I have access to my friend's art supplies, I use a combination of acetone (not nail-polish remover, but high-concentration acetone) and paint thinner (usually just more acetone, but that's not always the case) and soak one side of the tire for a few minutes, then start wiggling it until it pops off. Repeat for the other side. you can soak the whole tire, but it'll probably eat the foam inside (not that that matters either, as I've started packing the RC18T tires with bubble wrap, craft foam, or packing peanuts).

If you want to just nuke em off (If the tyres are shot completely with no hopes of getting them back to anything usable) then a soak in gasoline for about half an hour should have em in a condition to be removed from the wheel. (not recommended on painted or dyed wheels. Just plain colors)