How to get rid of an ex that won't give you up

Hey girlies I need advise, I have an ex that won't leave me alone. His gone back dating his ex but still giving me trouble constant threats and if he doesn't get a response from me he messageses my friends and family slating me off, more threats and I havnt been with him from 5 months and now im dating my best friend. My ex is a controller, theaf, cheat and lier and no matter how many times I tell him to leave me alone he doesn't listen and does everything to get in contact with me.