When people get banned, they go onto a list in our mod panel. In that same panel is our member-approval system. When new members (or returning banned posers) sign up to be admitted as members, the mods have to go in and approve or disapprove those new members. We click on the applicants' IP addresses and can immediately see if they have any alternate identities. Then we check those other identities to see if any of them are banned. If they are, we deny the membership. We have other ways, too, but that's our main way.

Some members in good standing have more than one identity, so we have a few duplicate identities anyway, which usually doesn't cause problems. It is possible for banned members to sign up for a new member account through a different IP address on a different PC or IP account so the IPs don't match when we cross-check them during approval, making the returnee seem like an all-new member instead of a returning poser. So occasionally returnees come back and slip under the wire that way. They nearly always manage to give themselves away, however, and get banned again anyway. Most of our chronic "bannees" have very distinct personalities, anger problems, and writing styles, which are fairly easy to quickly detect.

On a few occasions, banned members return with a new identity and new behavior, and we let them proceed, especially if they've been long-time members and they really do seem to have turned over a new leaf. As long as they're being civil, I don't personally have a problem with that. I can think of three of those folks off the top of my head who're active members now and are behaving themselves after a fairly colorful banishment history. I'm sure there are probably others if I think a bit longer.

Sorry for the long answer!