Well you could say in the license something about maintaining QBASIC compatibility

For example, you could say something like, "if a modification breaks QBASIC compatiblity, then the new package with modification shall not be called QB64"

Or you could disallow all modification or distribution for a fee but make the source available "for reference only".

I think that would mean that it would not allow QB64 code to be mixed with GPLed code, though.