I would disable signatures in the forum software for new users if I were you.
If they add to the conversation (so not spamming with “interesting”, “nice idea” and such bullshits) then they can stay, without posting their links it will turn out if they are there for the community or just for the link love or player poaching.
Might be a good idea to set up a new member group for your forum and every new member stays in that group until they demonstrate that they are not there only for the links and for the players. Disable posting links, signatures, etc for this group. Your players won’t post links I’m sure, so they won’t suffer from this barrier.
If they are spamming, then it is the same as other form of spamming, and get in touch with their aff manager.