Do not respond, it does take you down to their level. Then block them. It may be harsh, but they are feeding off your continued posting. Today, for the first time ever, I've blocked someone from my e-mail. Decided I didn't want any more contact. It's given closure, although there are other ways I can be contacted (not social media).

No one should have to put up with this in a so-called friendship. These people are cowards and need to confront their issues. Rant over!