I’ve been thinking postcard based account validation for online services might be a strategy to fight bots.
As in, rather than an email address, you register with a physical address and get mailed a post card.
A server operator would then have to approve mailing 1,000 post cards to whatever address the bot operator was working out of. The cost of starting and maintaining a bot farm skyrockets as a result (you not only have to pay to get the postcard, you have to maintain a physical presence somewhere … and potentially a lot of them if you get banned/caught with any frequency).
Similarly, most operators would presumably only mail to folks within their nation’s mail system. So if Russia wanted to create a bunch of US accounts on “mainstream” US hosted services, they’d have to physically put agents inside of the United States that are receiving these postcards … and now the FBI can treat this like any other organized domestic crime syndicate.