Problem with spam on forum 7.9

Is there any way to prevent spammers from putting spam on forum in forum 7.9?

I have tried all possible ways to prevent spammers from posting some odd posts, but without any luck. Spammers are posting there messages as guests. I have moved the topic, which is attacked by spammers to special forum and I restricted guest from posting on that forum. I did that for this forum (generally, so I changed the permissions to allow posting on that forum only to registered users). Also I changed settings for individual 'member' "Guest" to disallow posting new posts and replying to it (posting NO, edit posts NO, reply to posts NO), but without any success. This particural topic is still getting spammed. The restricitions are not working. Guests cannot post there post on that particural topic in that particural forum, but spammers, posting as guests, can. Is there any security hole in Web wiz forums 7.9 or what?

Why spammers can avoid restrictions and why are they still capable of posting spam on this topic as guests whether all the other guest cannot do that. I am asking again. Are they using any security hole? Does anyone know how to prevent that spam on forum 7.9 without upgrading to newer version?

I don't know any more, how to prevent spammers from spamming this single one affected topic on my forum. I have restricted guests from posting on that topic (more accurate: on the forum, where this topic is placed) and this apparently is not working. I also blocked IP adresses, but spammers are using IP adresses from all the world (all the spam has the same content, so they are connected with each other, but they are using IP adresses from many countries (maybe some sort of "evil" script is causing spam). I don't want to close this topic and I really want to get rid of that hidious spam on this topic, which has become almost unbereable.

One more thing. This spam has began few days ago only on one topic. Before that there was no problem with it.

If somebody knows the solution, please help.