A forum is only as good as its level of moderation. Unfortunately moderation takes up time which is a valuable commodity these days particularly as it’s done on a voluntary basis.
Spammers are persistent. They go to the trouble of joining the forum, sit back and wait for their moment. They generally target the most popular discussions or create a thread offering something “free” usually IPTV related. That’s guaranteed to attract big interest these days.
All that can be done is for genuine members to be extra vigilant and report any spam instances they see and obviously not click on the links. You’ll know by the activity of the member and the nature of the post what is genuine and what is not.