A Bot is advertising (alot)

(On my forum i have some "sub-forums" where you need to be registrated to make a post. However i also got one where you can post as a guest)

Im having problem with a bot posting random advertising om my forum. He/It also registrated some accounts untill i changed the registation.php, by adding the Timezone spambot-thingy.

Ive tried to ban the IP's, but thats no good since he/it is using a proxy. Also tried to ban the e-mail adress he/it is using, but i guess that only affects the people that tries to register.

I dont know enough PHP-coding to do something about it, so im asking you. Is there perhaps a way to make the registration.php timezone spambot-thingy to work with post.php? Or do you have another idea that could help me?

If you need any more info, feel free to ask.

Thanks