There's a nearly fool-proof solution: manually verify every submission.

You can use automated systems as a first line of defense against spam, and then hire people to manually verify every submission that makes it through. You can even use that as opportunity to ensure a certain quality of submission, even if it was submitted by a person.

Any legitimate submissions that get caught in the initial spam filter can use a manual appeal process (perhaps emailing and pleading their case which will go into a queue to be manually reviewed).

Sure, it's not necessary easy and submissions may take some time to appear on the site, but there would be essentially zero spam and low-quality content.