One way to remove ports is using
ports-mgmt/pkg_cutleaves.
Before you install a large meta-port with lots of dependencies you may want to uninstall (Such as KDE4) generate an exclude list (with -gl), when you want to uninstall just use
# pkg_cutleaves -x and press `y' a lot of times (Or use the -V flag).
I deinstalled KDE4 in a few minutes this way.
You didn't generate an exclude list, but pkg_cutleaves is still usefull, you just need to be careful not to remove any useful ports (i.e. pay attention and not mindlessly press `y').