make it immediately and publicly clear that a negative interaction is unwelcome
I couldn't agree more strongly... I mean we kind of have that already. Everytime I see something that has a score of like -40 because of all of the downvotes, I think they got their just punishment and it's clear that no one likes what they wrote. I think it's superior to replying because it doesn't give that person any reply to start an argument. Just silence and downvotes. But however we decide to do it, i think we should be very open and upfront with what's expected behavior. And I'd like to see that happen more often.
switch off visibility for posts/comments that have received a certain proportion of downvotes
PieFed has that feature. Comments with a score less than -10 (I think) just collapse. I think we need more of those features in Lemmy and the respective apps.