I keep one slot open for new people to register and ask for permission to regular forum discussions,he/she registers under different names and IP addresses then spams with porn.I'd like to report him to police but he's using different IP addresses everytime.Anyone able to help?

Report the spam to our SIRT Team by posting a copy of the spam here:

/modules.php?name=SIRT&fp=queue

Make sure you indicate at the top of the report that this was comment spam. That way they know it was posted on a web site not sent as an email or IM.

As to doing anything about the posters, well, they may very well be zombies, or use anonymous proxy servers, so trying to go after IPs or unames of posters is generally useless. You need to report the spamvertised site, and hope that our SIRT Team can accomplish a site take down, and that the ISP hosting the site will be cooperative.