In my forumd all new users they do is register and start posting advertisements of their products all over the forum. Users should not be allowed to post any threads unless they make x amount of post on the forum. By post I mean, replies to previous threads etc. Once they engage on the forum and start replying to threads, they can start new threads.
Restrict new users by group permission by restricting the
Can post new threads permission. Then set up a group promotion to promote new users to another group without these restrictions - after, for example - X number of posts.
You may also find this plugin useful:
https://community.mybb.com/mods.php?acti...w&pid=1264
We use Crazycat's plugin effectively to combat this problem of unauthorised product promotion. Spamming I'd call it. No referral traffic - they soon lose interest.