In the 1980's, we kept anklebyters off dial-up BBSses with a simple technique: voice validation. To join the forum, you had to fill an application first, which included your real name and phone number. The sysop would give you a call for a quick chat, and then grant you access if you didn't seem like a twit.
This would be entirely practical for some small-time operator trying to run a forum off residential broadband, while impractical for the reddits, facebooks and twitters.