Modelling is the best solution. Assuming they are not deliberately harassing, giving a kind and empathetic reminder to the contributor that this is place is optimized for kindness should be all what it takes. I don’t know if the platform allows for private messages, but sometimes a kind private message is all what you need to curb these behaviors. A short note explaining and allowing them to edit their message should be enough. Private messages are best because it prevents the person to feel publicly humiliated, so they have a chance to save face. If it has to be be public, then it is a matter of being super careful when pointing out that they weren’t kind.
Even if the person who wasn’t kind doesn’t get it, other people will pick up on the kindness given. And that will help cement the culture of kindness.
The site sort of looks like SO, so many people will unconsciously use the SO behaviors. A lot of dev communities run in a Lord of the Flies fashion, so people coming from those communities may slip now and then. This is particularly the case with beginners who desperately want to be “real” programmers and whose only role models they have run into are people engaged in condescension battles.