Unfortunately the forum doesn't have a feature where we can add captcha or other filter so that we can prevent these kind of spam users and messages to register.
The forum on which I'm a moderator uses SMF forum software which is the same as this forum. The registration has an option to provide a captcha module. I'm fairly certain the admin of the forum does not pay for this although it may be an optional (no cost) add-in. While there are some dedicated spammers who have lots of time on their hands, since we implemented this, the number of registrations by spammers has been cut by more than 90%. We used get get about 10-20 posts a day, but we now get a couple a week. Our forum uses SMF 2.0.17 while AM's version is 2.0.15.
Perhaps it's worth investigating.