I've never maintained a friendly relationship with an ex. When it's over, I just cut all ties. The last girl I broke up with admitted cheating on me, so I called her a few nasty names and walked out of her place.
She tried texting and calling me, but I never responded. She would even find out what I would be doing through mutual friends and just show up where I was hanging out. She'd try to say hi, but I would completely ignore her.
After a couple weeks, she got the point, and I have never had contact with her again.
My advice to you is to do the same.