FreeBSD ports: remove build dependencies after installing port
Solution 1:
My suggestion would be pkg_cutleaves (available in the ports collection), or pkg-orphan -- Both of these do the same basic thing (list packages that aren't referred to as a dependency by any other package, and ask you what to do).
I wouldn't bother though: The build dependencies will just have to be rebuilt and reinstalled when you upgrade the port, and they generally don't take up huge amounts of room. Leaving them won't hurt you unless your system is severely disk-space constrained.