Comments validation based on links number

Hi,

I have searched for this topic a lot but without success. I hope you guys will be able to help me out.

I am getting a lot of SPAM comments in my pending comments folder. Most of them contain a lot of links. I want to add a validation to the comment form, so that when a comment has more than 2 links, instead of being posted (and end up in pending folder), a message would be displayed that it is forbidden to post comments with more than 2 links and the comment would be denied.

How can I do it? Is there any plugin for it? Or maybe I need to change some php?

Thank you in advance for your help!