I need help writing a plugin for comments

In order to help prevent spam I have hidden the URL field in my comments form, but I am still getting a ton of spam that has the URL filled in. So I can only assume that these are bots that are automatically submitting to my wp-comments-post.php file. Since these are all going to be spam I would like to write a plugin that rejects any comment that has the URL field filled in.

I tried looking at the wp-comments-post.php and comments.php files for hooks that I could use, but I did not find anything useful. I am probably looking in the wrong place.

Does anyone have experience with this and can you point me in the right direction?