[peterska2]

Are you using image and email verification in the registration process?

If you are using these and guest posting is disabled, then you may want to consider the step of moderating newly registered users posts. The best way to do this would be to create a new usergroup with the same permissions as your registered users group, set up a promotion to move them to that usergroup if they have x posts (say 2 or 5 - most spammers never stick around that long), run the user promotions cron job, then change the permissions on your registered users group (usergroupid 2) to moderate all posts. It is more work for you and your staff, but coupled with an announcement it stops all spam (and the recent meta refresh exploits) as the posts do not show up until they have been approved. You can delete them without them ever getting onto the site, which is a big plus.

You may want to consider the quickmod modification in conjunction with moderating posts as this then means that you don't have to keep going into the ACP or modcp to see if there are any posts awaiting moderation. You can just see at a glance under the navbar everything that is in the moderation queue.