I can use something like "Object.send(:remove_const, :Game_Player)", that's not the problem. As F0 said, we need an automatic way to do this.
If you can't find anything, I'll try to find that part in the Ruby source, just delete it and compile Ruby without it. Or I can add the constant removal there, it doesn't matter. Though, I'd like to avoid poking around in Ruby's source.
EDIT: Alright, I have simple commented out that piece of code and recompiled Ruby. Task finished.