FIrst find out which desktop manager do you use. Switch to ttyX (CTRL+ALT+Fx). Do it via `ps -Af`, you should see one of following: startx, kdm, xdm, gdm. Then killall the one you have seen. If it is startx, kill should be enough, but the others spawns their own child process, so you should kill the whole process tree.
And if you use startx to bring up the graphics, CTRL+ALT+BACKSPASE should be enough.