When it comes to killing a frozen process, Ubuntu and its derivatives offer a great variety of ways to do so:
You can open the System Monitor to end or kill a running process. Just right-click and annihilate:
Then there is the Force Quit applet. Not that powerful, but does its job in most of the cases:
Working similarly but more powerful is the xkill command line tool. You can also wrap that one into a launcher and use it the same way as an applet.
Another command line tool is killall followed by the name of the process, granting the advantage of not having to know the process ID of the program, which the next tool requires:
kill, provided with the -9 switch and the ID of the process (available with top or ps) shuts down nearly every running process.
If none of those commands help, there is still the option to close the current session with ctrl + alt + backspace. This however comes with the warranty of losing any data you could not save before!