The best way would be to use yum grouplist to see the installed groups on your machine
they are the ones under the heading "Installed Groups:" (the other section lists the groups that are available but not yet installed)
the group you want to remove would be KDE Software Compilation
so use groupremove to take it off:
yum groupremove "KDE Software Compilation"
The reason that using grouplist is a good idea is because the KDE people keep renaming the thing all the time so this way you
get the current name. If fact I'm not sure if Fedora 13 has that group called "KDE Software Compilation", I'm just calling it that
since that's what my Fedora 14 shows.