This has some similarities to the invite-tree method that lobste.rs uses. You have to convince another, existing user that you're human to join. If a bot invites lots of other bots it's easy to tree-ban them all, if a human is repeatedly fallible you can remove their invite privileges, but you still get bots in when they trick humans (lobsters isn't handshakes-at-doorstep level by any margin).

I convinced another user to invite me over IRC. That's probably the worst medium for convincing someone that you're human, but hey, humanity through obscurity :)