To delete an auto-tag, log into your dashboard, and navigate to the Auto Tag tab along the top menu.  

Under the “MANAGE” tab, click “edit” to the right of the tag you wish to delete. Scroll over the down arrow menu to the right of the “CANCEL” button, and click “Delete this tag”.

Doing so will delete the auto-tag and remove the tag from all feedback that it has been matched with.  There is no way to recover a deleted tag, so please do so carefully.  

NOTE: If you use "Disable auto-tagging", this will stop the tagging of new feedback but will keep the tag applied on your old feedback. This can be an option if you just want to stop new tags from being applied and not completely delete your tag.