Why won't my ex just leave me alone?

i broke up with my ex boyfriend about 10 months ago, and he still won't leave me alone, he is always trying to contact me and meet up with me it got that bad that when i was staying with a friend he sat outside on the doorstep for 5 hours knocking on the door, my friends mother had to get up at 3 am to get rid of him, but he never left, i felt so bad. on my birthday which was just over a month after i finished with him, he came to my house with expensive presents hoping to get me back, but i told him i didn't want to go back. he did and still has a seriously bad drinking problem... if he drinks too much vodka he can turn violent, he found out that i was getting involved with another man and he was extremely angry. i am worried he might do something horrible. i have been ignoring him but he is still persistent, i don't know what to do.