Is there any way to remove build dependencies of the port after installing it?
Many ports use exotic tools for building and these tools are not used anymore when port is builded and installed.
My suggestion would be
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.