check /etc/rc3.d/ and /etc/rc5.d/
Assuming both start X, I'm assuming both have a link to xdm, kdm, etc. First check that this is just a symbolic link (presumably to /etc/init.d/xdm). If so, then just remove the symbolic link, and xdm, kdm, etc. won't start and you'll be met with a tty login.