CategorySelect is a tool enabled by default on Wikia in the Oasis skin. It allows users to add categories directly from the article without going into edit mode. This causes problems when registered or unregistered users use it to add random categories, sometimes confusing it with a plain Article comments form!
It only gives option to add categories, not to remove or edit them, so it's basically useful for people that want to use it to earn achievement points doing a handful of quick edits by this system.
How to disableEdit
Disabling only for you Edit
You can disable it from Special:Preferences, check the Disable Category module (only applies if visual mode editing is disabled) preference, under Editing tab.
Disabling it for anons Edit
Add the following code to MediaWiki:Wikia.js on your wiki to prevent the category select controls from appearing to unregistered users when viewing an article (not when editing).
Disabling it for all users Edit
Putting this CSS code in MediaWiki:Wikia.css will hide the controls for all users when viewing an article (not when editing).