Hello
By "some of them" are you referring to "sessions" or alternative Window Managers? Both GDM and KDM get their cues from XDM which draws from XSessions and Xresources. Poking around in these will allow you to delete the call for a particular WM but not the WM itself. The WM, if installed, must be uninstalled through something like KPackage but that won't remove the name from your login manager. You have to do both if you want to be thorough. BTW IIRC some versions of Gnome and KDE have graphical configurators to alter the login manager. Since I'm not sure what you mean, I can't know if this helps.
Jimmy