Can't get rid of crazy ex

I broke up after my ex boyfriend from my old uni four months ago mainly because I didn't like him anymore but four months on he still won't leave me alone! Most of the time he just calls and texts a lot and he can be quite pleasant but then just randomly turn on me and get really angry and threatening and nasty; it's quite upsetting a lot of the time. I usually manage to calm him down after a while but it's so stressful and really preventing me from moving on. I would just block him and change my number but he has some nude photos of me (stupid i know) that he's threatened to send to my family (whether he would is doubtful but I don't want to risk it) so I can't exactly just give him the cold shoulder. I have made it very clear that I'm not interested but he just won't leave me in peace! I was hoping that he'd just lose interest after a while but he's totally obsessed with me and I don't know how much longer I can deal with the burden of it all :( . Id really appreciate some advice on how to deal with the situation.

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