Forum spam is a pervasive issue on just about any forum, that can be very hard to deal with. In general there is no hacking or backdoor that is involved. The spammers simply register and do their deed. As a volunteer moderator at a computer-related forum I’ve even seen numerous cases in which the spammers use automated software to do that, frequently re-registering under different screen-names, using different email addresses.
In addition to advertising of bogus products or services, spamming is often done for purposes of SEO (search engine optimization, meaning elevation of the placement of a site in search engine results). That is often done not by anyone directly associated with the site for which placement is being optimized, but by third party spammers who probably receive a few cents for each link to that site which they place on other sites.
There are no easy answers, and I commend the moderators here for deleting the offending posts in a reasonably prompt manner.
An additional approach that can sometimes be appropriate, if used judiciously, is to block IP addresses from being able to access the site, using wildcards for the last few digits (e.g. 89.71.***.**). Perhaps the moderators are already doing that to some extent. The risk, of course, is that legitimate registrants may be blocked in the process.
Another possible approach is to have posts submitted by new registrants subjected to moderator approval before being made visible on the site.
Regards,
-- Al
