Simple and easy suggestion: above the textbox on the reply page, add some guidance on tone and behaviour. Like,
'Please refrain from making mean-spirited comments,
we like to maintain a positive atmosphere at HN;
and if you are planning to crack a joke, you might want
to think twice as jokes here are usually downvoted unless
they're *particularly* amusing.
Or whatever you think's more appropriate. The problem to me seems that general bitchy behaviour is the norm on internet IT forums, so people come here thinking it's ok. Maybe they just need a little guidance.