one of the now closed down mini forums i used to visit had the same problem, especially with several repeat offenders using different usernames but still from the same company, one offender made it at daily habit to post at least 1 post advertising their wares in every thread, so eventaully after a few emails to the company concerned about and a few unkind words back, they were simply spammed right back everyday over 1000 emails got sent to their sales email address with every email filled with 10kb of random letters with "stop spamming XXXXXXX forum" in the subject line, after 1 week pretty much all forms of spamming with odd exception stopped,
no amount of blocking ,filtering, questioning ,moderating is ever going to stop it,