You can boil them, bend them back into shape while still warm, then run them under cold water to lock it back in.
You can actually add a bit of reverse bend in them that will help them not to bend back so soon.
Aluminum arms on any RC except for drift and on-road cars, where the arms are tucked under a body, are not good man. For open wheeled cars, it is a bad idea. Aluminum bends and breaks, and is expensive to replace. Plastic arms will actually take more abuse. I learned this from experience, as...