Some possible solution for this :
1) Asking for email verification. Will reduce registration with bots.
2) Deleting old accounts with no activity.
3) IP restrictions being made more tough.
Problem with these steps is that it will affect genuine users registering on the forum as well.
Wont possible theymos doesnt want these email verifications as well as KYC for the members,deleting old accounts with no activity wont change anything.
IP Restriction could be bypassed by any VPN provider i personally use one so that i would stay anonymous atleast you can also use other browsers like TOR.
Most of the new accounts coming from those members who wants to earn more money with these signature campaigns which is the main cause of spam in this forum.