I have a source saying positive commands usually occur in the perfective aspect while negative commands usually occur in the imperfective aspect - which is valid here :D.
Not really, that sounds very strange to me. I'm not sure why, but I think that if you use imperative to say "don't do this", it's like "do not do this at all", not "do not do this at some specific occasion". So in that way it's imperfective.
Is not 'Don't talk to him' the same as 'Do not tell him'. My translation of 'Don't talk to him' was rejected.