This is something that could've been put into the Forum Suggestions thread as it would affect other parts of the site as well (comment boxes, dedicated forums on here) as well the general prevalence of the issue I felt giving this suggestion its own thread would be more appropriate.
Itch.io is an extremely open distribution platform and community, the most open one around - and that's good. However this openness has sadly been exploited very heavily by spambots due to the lack of anti-spam system. Most of these are simply advertising their service or site which is benign but still somewhat annoying. Some of these posts however are venturing further out and are even offering illegal services - the latest spam post I noticed on here for example is selling fake documents and counterfeit money. (Link to said post)
What I'm suggesting is an anti-spam system that stops spambots in their tracks during sign-up. Something like a CAPTCHA would be good, or Google's reCAPTCHA system which is even better (reCAPTCHA just involves clicking a checkbox rather than typing in blurry/distorted words.)
This has been successful on many other sites and it should be successful here as well.