You cannot remove your account, due to "certain limitations". However, there still is a lot you *can* do, which is listed on this site: http://help.wikia.com/wiki/Help:Closing_an_account - so Wikia gets a grey listing here.
To contact them, you can use this form: http://community.wikia.com/wiki/Special:Contact/general
If you signed up for Wikia using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Wikia. Click the icon below to find out how.