adamhm: Keyboard shortcuts: http://www.shortcutworld.com/en/linux/Linux-Mint-Cinnamon-Edition_17.2.html Shortcut for 'xkill'
I recommend setting a shortcut for this, as it is very useful and it saves time having to load the System Monitor to terminate something if it locks up & won't close normally.
Go to System Settings --> Keyboard --> Shortcuts
Click "Add custom shortcut", name it however you want, enter "xkill" in the command box and click "Add".
Then select it (under Custom Shortcuts) and click on "unassigned" in the Keyboard bindings section to set a keyboard shortcut for it (I use Ctrl+Alt+X).
When you press the key combination you've set your mouse cursor will change to an X (or a skull & crossbones or similar, depending on mouse cursor theme) - you can then left click on any window to forcibly terminate it. Right click to cancel.
One other possible use of this facility: If you like to take screenshots, try setting a shortcut to scrot in the same manner. Now when you press the keyboard shortcut you chose, a screenshot will be saved, probably in your home directory. (Try it and report back where the screenshot is saved.)