[pllbnk]

Maybe some proof-of-work scheme where uploading content would require the uploader to solve a cryptographic puzzle, hence reducing overall number of posts? The PoW difficulty should somehow be correlated to economics where it wouldn't be too expensive but also wouldn't make sense to do mass uploading via bot farms?

[looselygoosy]

Like a CAPTCHA?

[pllbnk]

Good one, honestly didn't think about it. But visual or other kind of human-accessible captchas can be solved by bots. My suggested PoW would be computational.