You can always open a console unless your system is totally screwed up.
If your GUI freezes, get into a root console on another screen by going to runlevel 3. CTRL+ALT+F3.
Now if you know the name of the process that you think is causing the hangups, do this:
$ ps faux | grep <processname>
it will return 2 lines of info. The first one is the process you're trying to terminate, and the second should be the process of your search. It finds itself, basically. Anyway, look for the first 4-6 digit number on the first line. That should be the PID.
type:
$ kill -9 <PID>
If it says no such process, you picked the wrong number. Keep trying until you get nothing returned. That means the command was successful.
that's how you manage rogue tasks in Linux
Once you have killed the task, make sure you type "exit" to get out of root in that shell. You don't want to leave it running with root in the background.
Now CTRL+ALT+F5 oughta get you back into GUI mode. If not, go back into runlevel 3, login as root and type "reboot"
That'll usually bring your system back happy.