It looks like
pkg remove <pkg> is able to remove a package installed via ports. But I didn't find any packages built by typing
make in the ports dir. So what's the magic here? How does
pkg remove know about the files belonging to a packge?
make install in the ports collection registers the package to the FreeBSD package system (by creating a temporary package manifest from the port and running
pkg register). From this the
pkg command knows what is installed and how to remove it.
a simple make install will install the application and register it with the package system.
It is also stated in the FreeBSD handbook that you can use pkg to remove installed ports.
Installed ports can be uninstalled using pkg delete.