How about just banning ICO announcements and their sig spammers.
It is the ideal step but very difficult to achieve. Primary goal of every website is to achieve traffic.
More traffic on website means more money. If bitcointalk ban ico postings, users will start posting on medium or any other forum.
Other forum will become hub for ico and alt tokens as well.
Ppl will start moving there and it will become a threat to this forum itself.
After all these ico posts are generating huge traffic for bitcointalk, and for those ico ppl are purchasing copper membership as well.