Sometimes you may find that XBMC add-on cannot be uninstalled normal way via System > Add-ons as Uninstall option is greyed out. This often happens because add-on is installed in a system wide directory. In this case add-on can be deleted manually from the add-on directory. Location can vary depending on your Operating System and version. Couple of examples below:

• Windows 7 32 bit: C:\Program Files (x86)\XBMC\addons
• Windows 7 64 bit: C:\Program Files (x86)\XBMC\addons
• Ubuntu Linux: /usr/share/xbmc/addons