How do I uninstall a font?
Mac OS X places installed fonts in the user's Library folder to differentiate between system fonts (located in /System/Library/Fonts), OS fonts (/Library/Fonts) and user installed fonts (/Users/~/Library/Fonts).

You can change this by open Font Book preferences (File>Preferences) and changing the "Default Install Location" option from User to Computer. 

Hope this helps.

Added on 1/12/12 at 2:35pm:

Forgot to mention - If you're not able to delete the font from Font Book, try removing it from the /Users/~/Library/Fonts directory and emptying the trash. Also, make sure it's not a font that is also in the /Library/Fonts directory (preinstalled).