I counted at least 33 spam posts posted in the past 12 hours; all of them were created from unregistered users. Maybe unregistered users should not be able to create posts, in the same way they are not able to post questions on Stack Exchange. The reason for not allowing unregistered users to post questions on Stack Overflow is the high number of questions already posted; in our case, the reason is the opposite: We don't have enough active users that flag spam, and it is difficult that spam posts even reach 4 flags, which means they are not automatically deleted.
Update: We are back with spam posts created from unregistered account.
If requiring to have a registered account to write a post is not an acceptable solution, then it could be the case of not allowing to a user to write a post that just contains a single URL, or where the ratio between words/URLs is lower than a value (which means the post contains more characters for URLs than characters for other words).