To avoid the latest problem with blog spam, I've created a new usergroup called "Confirmed User", which have the same privileges as the old "Registered users" group. Members are automatically promoted to the new group once they've been registered for at least 10 days AND posted 10 posts in the forum.
The privileges of the old default usergroup "Registered users" have been nerfed in that all blog post are put in the moderation que, they can't upload files to the download section and can't use links in their signature. I have also pruned a lot of users with no post that haven't been logged on for a long time.
This should help reduce the spam-problems we've been having, without hindering the active members of the forum. Hopefully.