As I recall, when I made a metafilter account many years ago (2007!?!?) it costs 5 dollars to join. Which is a low enough amount that people can do it casually, but it's enough to deter people making random sockpuppets for the purposes of baiting and trolling and spamming. Introducing a barrier like that is an interesting and I think very valid method to maintaining the quality of an online discussion forum.
It's still $5 but they say on the signup page they'll waive it if you ask because you can't afford it/can't send the money somehow. So it's still very accessible but very, very few spammers bother to do either (a few have in the past decade and a half, of course).
