So, simply, press Alt+F2. You'll get this dialog box:
Type in 'xkill' and hit Run. Now, your pointer will change to a little cross. Aim the window that's frozen up and shoot it down. You're done. The program will close down.
As another little tip, you can even add a shortcut icon for this task killer to a Panel, like so:
|See the window with a crack?|
In the field there, type in 'xkill' and add it. You're done. Now, you are armed with a gun to shoot down unstable apps with, anytime.
Beginners can check out my recent post on how to log into Terminal as root, and those who are dual-booting Windows with Ubuntu can see how to switch from the very cluttered GRUB bootloader to the Windows bootloader over here.