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.