How do I prevent comments?

I am forcing users to register before comments can be placed. I recently had comment placed on articles on my site with google analytics scripts and styles modifying my widgets to turn them into banner adds and to link back to another website. I caught it because it got held for moderation, which was disabled.

Moderation is disabled
Registration before commenting is enabled
and I checked to make sure you have to log in before posting a comment.

There is no user related to this comment, considering I only have two actual users besides my self and I know them personally its easy to verify it was not them. What is going on?