You might need to add a plugin (or add your own comment filter) to 'count' the number of URLs in a comment before it is stored.
The Akismet plugin is a good one for catching a lot of spam. For one the sites that I managed, I had a similar problem. I ended up writing a plugin to allow for counting URLs (and lots of other customization of the comment form). It's called "FormSpammerTrap for Comments". It is fairly effective against someone that fills in the comment form.
It is not so effective for someone that uses CURL to directly post to the wp-comments.php page, bypassing the comment form. I haven't found a good way to block that yet; there aren't filters that happen in the proper place to block CURL commenters.