They certainly are legally enforceable and some software does this already. I can't recall exactly which software it was, but I know I have encountered multipl

They certainly are legally enforceable and some software does this already. I can't recall exactly which software it was, but I know I have encountered multiple times software licenses which forbid the softwares use by military organizations or for military purposes, or which forbid government use. Licenses can contain practically anything you like. And thus far, courts haven't been restrictive at all in terms of what can be included.