How to Deal with Ex's Creepy Intimidation Techniques?

I went to a friends birthday party and my ex was there (we don't get on or talk; he treated me like dirt when we were together). I stayed out his way; but I could feel him staring at me. My friends said that when they spoke to him, he would move himself so that I was in his view.





A few days ago it was another friends birthday, and he was there; I didn't talk to him. A mutual friend 'Bob' (who I was dating briefly ages ago) said my ex came up to him and said, 'Why is (Me) here?' (I had been invited my the birthday girl, and was friends with the majority of people at the party)





Bob replied, 'Well we're all adults.' to which my ex replied, 'Well she cheated on you with me.' Which is a complete lie!





I was irritated that he had tried to upset my friend, but realized he was just being petty. I continued to ignore him, but later on I was sitting talking to another friend, and he proceeded to stand directly behind my friend, facing me, and stare at me for about a minute. I refused to make eye contact but noticed that later when we went to a club he was staring at me again, for ages.





Why is he staring at me? I assume it is to make me feel unwelcome or to play head games with me;





Can anyone tell me how to deal with his childishness? Should I continue to ignore him or make polite conversation despite the fact he's nasty to me?How to Deal with Ex's Creepy Intimidation Techniques?
Its normal for guys to do it after break ups most of the time ( do you live in a small area?).