Well as someone who has recently had to get the police involved due to harassment from DPs ex, my recommendation would be along the lines of what the police told me.
Stop any contact by both of you. Find an intermediary who can communicate if possible and get a court order if needed. If you do respond to any communication, only explain that it is unwanted contact and that you want it to stop.
We have blocked 11 different numbers and countless Facebook profiles, but 5 years on the abuse continues, as do the 4am phone calls. This is why I involved the police.
Not that it matters, but she left DP, her husband for someone 16 years younger and they had been separated about 11 months when we met. Didn't stop her rage towards me though. I wasn't allowed to tell her children what to do, even though they were staying in my home. She has withheld contact on many occasions. She does her best to turn her children against DP and definitely against me. She is a pathological liar and I will no longer be in her children's company on my own, as frankly it puts me at risk and I have our very young DC to think about. They have lied about things in the past and their mother would love nothing more than to drag me through something like that.
I haven't seen DPs children in 7 months and although it's sad, given the state of the situation at the moment, it's definitely for the best. I understand he needs to try his best to maintain a relationship with his children, but that doesn't include me.
We have had so many arguments as DP thinks that we should just accept her bad behaviour and play her games and run around after them all for the sake of his children, but it was impacting our lives. He has finally realised I think just how bad things were and how much stress and upset she was causing and I live in hope that it's largely all over now.
DP is lucky he has a way of keeping in regular contact with his DC without involving his ex, which I understand isn't possible for everyone.
Good luck OP. It's been hell on Earth for me and I've only stuck it out because of our children.