I never used to write negative refs on CS - it was either positive, or nothing. I think this is partly because I was very picky about who I hosted/surfed with, so generally I didn’t meet people who I spent time with that was bad enough to warrant a negative ref. I also grew to be sceptical of “five star” reference culture and its strange distortions, both in CS and elsewhere.
However, if it came to it and it was, for example, a safeguarding issue, I would leave negative refs on Couchers for people who I felt would have a serious or significantly deleterious impact on the community at large.
I would include details to justify my reference and a note of warning to others. Personally, I wouldn’t include anything that states that people shouldn’t host/surf with that particular person, but just that they have been warned and should think carefully before they engage with them.
Of course, if there was somthing really serious, I’d flag it right away.
The only time I left a negative ref was for a host whose place was so dirty it had rats in it. “Nice guy, but shame about the rats. I chose not to stay and if you don’t like rats either, you might be advised to find someone else to stay with.”, or something to that effect.
Side question: is flagging part of onboarding atm? Would be good for new users to experience it and also get some kind of taste of what kind of response from Couchers they might expect.