There is proof of stake. You stake some moderate amount ($25), refundable at any time but forfeit if you spam. For a non-spammer it effectively costs nothing but the spammer loses their stake for every spam someone reports, and every spam posted against the same stake can be disabled at once.
The requirement obviously being the ability to make small anonymous payments.
I think it would be great for Twitter if they could get away from the advertising model. If holding a Twitter account required holding a share of Twitter stock, I wonder if users could end up owning the platform?
The requirement obviously being the ability to make small anonymous payments.