do not engage
I got a bitter and nasty email this morning that kind of threw off my whole day. I don't, generally, like it when people are mad at me, especially in a circumstance like this one where I am pretty much entirely confident that I have done nothing wrong. In this case I was furious about the whole deal and had to stop and calm myself down before proceeding.
Not responding might be the best option - and usually is if it's a social media stranger - but this was someone I know and I have never much liked ghosting people.