Soaking the rims in acetone works for me. I have tried boiling them & just made a mess.

Foam soaked off in acetone can be reused. If you chunk a tyre & the rim is still good you can get rid of the foam & then take the good foam off a rim that had had the centre punched out & reuse it. Re glue the good foam to the good rim with shoo goo. I also soak the foam off my worn out 1/8 rims & reuse on tourers. The 1/8 to tourers I use as practice tyres.