It seems like there are always some people who like to cause problems, and forums aren't different than other venues. Forums attract various types, including spammers (who are promoting something), harassers and malcontents. The latter usually have some kind of axe to grind, and pick on other members or, like spoiled toddlers, act up to get attention.
Most forum software includes features like member banning, domain banning (to prevent registration from specific domains), IP banning, etc. And if a user keeps getting their email address banned, one would think they would tire of the game and go somewhere else.
On some forums, though, a large number of users register with free email address from yahoo, hotmail, and the like. I have a board that attracts a lot of older teens, and I'd guess that 90% of this demographic uses free e-mail. Typically, they use their free e-mail to express some individuality, plus they often have little recourse. They can't get a school address, and their parents use their home ISP address or addresses.
International users often end up using free e-mail accounts, too.
I've encountered forums that don't permit registering from a free e-mail domain, but this seems quite impractical based on my correspondence with some of these free e-mail users.
One thought that occurred to me was requiring a very nominal registration fee for users from free e-mail domains, but that would probably just spark a hunt for obscure domains that aren't obviously free. And a registration free for everyone would definitely cut membership and participation.
At the moment, we just do our best to spot problems and disable those accounts. Of course, the same day that problem individual can be back with another addy. Or, he might have stored up a few accounts already. If he's using a dynamic IP like AOL gives, IP banning isn't of much use either.
Has anyone found any clever ways of dealing with free e-mail proliferation and policing forum registration?