How do I keep QTKitServer (QuickLook Helper) CPU usage from exploding out of control? OR How do I disable QuickLook entirely?
Open Terminal, type:

sudo nano /System/Library/Sandbox/Profiles/com.apple.qtkitserver.sb

This will open the nano editor with the rules file. Scroll right to the bottom of this file and enter:

(deny file-read* (regex #"^.*\.mkv$"))

After this do Ctrl-O and then Ctrl-X to save and exit the application.

Reference: http://forums.macrumors.com/showthread.php?t=1463364