how to mark llvm* forbidden?

[Chris H]

OK I'm chasing -CURRENT, and I performed an initial
install, followed by a new world/kernel && ports about a
mos ago. Last Friday, I svn upped the system (src && ports),
rebuilt/installed world/kernel. I just began rebuilding
the ports, only to find that when finished, I will likely
end up with every version of llvm && clang from version 3
to the now current 4. My build session is currently tying
nearly every core on the CPU with llvm builds. Given that
llvm4 comes in base. Is there *any* reason I can not insist
that the ports I upgrade, or build, just use the version(s)
of clang/llvm in base? If so. How do I inform the ports
that they may *only* use the version(s) in base?

Thank you for all your time, and consideration.

--Chris