you can't just straight decompile a map, there'll be major brush errors and the decompilee will have to put a lot of effort in to even get the map playable.
There isn't a hard and fast way to stop people from doing it, either respect your fellow mappers enough to not steal your work (I've never seen a case of it in 6 years of mapping), make the map so damn complex the decompiler just crashes (this ususally happens concerning my maps
or try compiling a few wads in, winbsp doesnt like this very much.
Other than that there isn't much you can do, sorry!