Long before cryptocurrencies existed, proof-of-work was already being used to hinder bots. For every post, vote, etc., a cryptographic task has to be solved by the device used for it. Imperceptibly fast for the normal user, but for a bot trying to perform hundreds or thousands of actions in a row, a really annoying speed bump.
This combined with more classic blockades such as CAPTCHAs (especially image recognition, which is still expensive in mass despite the advances in AI) should at least represent a first major obstacle.