This question already has an answer here:
I used to believe that captcha-like solutions are the websites' primary way to combat spambots. I used to believe that any site that doesn't implement such solution is likely to be flooded by shady weight loss pills ads.
But this is not the case. There is a counterexample. The Pokemon Showdown website allows unregistered people, who haven't attempted to pass a captcha, to freely talk in their Help chatroom as long as they choose a nick. From my experience this chatroom is not being flooded by shady weight loss pills ads.
According to people knowledgeable in this site:
Most ways for bots to connect to PS are blocked automatically
Ah. So bots do try to spam, but are blocked if I understand correctly.
OK then. What are other, non-captcha ways to block bots?
Note I bring the PS example only to back up my point that it is possible; but I'm asking how can this be done, not how PS does this; so please don't close this question as off-topic and only answerable by PS staff.