Crazy Ex

I split up with my short-term boyfriend (we'd only been dating for a few months) over a year ago but he won't leave me alone. The break was messy and after a few months of trying to be civil (mainly because we shared a lot of the same friends) I decided to cut all ties and didn't speak to him for around 6 months. Then recently he invited me and all my friends to a party... Because I'm the most organised I was in charge of plans so ended up speaking to him (mainly over social media) exclusively about it. The plans failed but he kept on messaging me and I then found out he had plans to get me back on Valentine's Day. I decided I needed to have a conversation with him and made it very clear I had no intention of getting back together with him and he seemed to get the message - particularly given that he knows I've had brief encounters with various people since we broke up. However he still kept on texting multiple times in the day and gradually started coming up to me all the time during my frees, break and lunch, trying to arrange to meet all the time. I decided to change tactic and went back to ignoring him. But I slipped up and accidently answered an out of the blue phone call just over a week ago not realising it was him calling. Since then, despite my continued effort to ignore him, he still calls me at least once a day and messages me even though I don't respond! When we were dating he would call me 7 times a day sometimes even though he knew I was at work and so has always been a bit clingy but this is ridiculous particularly given that I've made it very clear that I'm not interested. I don't know what to do?!