I have tested two different plugins based on the same concept - spammers list - http://wordpress.org/plugins/avh-first-defense-against-spam/ and http://wordpress.org/plugins/stop-spammer-registrations-plugin/. If someone is blocked, it should show a message.
They look to work nice but some of my visitors complain they could not post comment. They saw a "mysterious red border square" and their comment cannot be sent - the same problem with the two plugins and on two different sites with different templates. And no warning message.
The solution was to add their IP on whitelist but it was strange I cannot explain this problem.
The last visitor with problem help me to make some test and it seems that BBQ was the problem - after desactivation, the visitor could post.