Comment Spam security hole

Hello,

I am using wordpress 2.0.5 and facing a strange problem on my site, I have installed Math comment spam plugin and set the option that only registered user can post comments but I have seen that these both options isn’t working and I am getting lots of spam. It is not registered users posting it . These comments are not from registered users and comming from same IP so there must be some holes in the script that were exploited.

This is what happening with me. I am not sure how to stop them and all spam plugins are failed to stop them too.

Does any one has solution for this do I need to upgrade?

Thank you