This behavior always bums me out. Seems like HN of all places should accept and encourage disagreements as long as the resulting conversation is interesting and doesn't devolve. Using the down arrow to try and stifle opposing views is disappointing to see.
Once told someone "good luck" on a project idea and it was upvoted to 2 and then a few hours later I noticed it had been downvoted to 1.
I pondered what kind of community HN was that day...
I think the issue there is that 'good luck' doesn't add anything to the discussion, so someone decided it wasn't 'worthy' of the upvote.
Anything, including jealousy is in human nature and HN has a large enough footprint that you can reasonably expect that if it is in human nature that it will have found it's way here.
I don't think you can stop that unless you put a hard cap on membership.