Say everything you need to say to that person, anything and everything you need to say to get the closure you need. List all of the reasons that the person is bad for you or the reasons you’re better off without them. Cry about it, as much as necessary or as much as physically possible. Then delete the person completely from your life, block their number, delete them off all social networking sites, delete all pictures/screenshots/songs that remind you of them. Then do things that keep you busy, delete all your music and download a whole new set of songs, redecorate your room, take up a hobby. Just anything that takes concentration and will stop you from being sad and depressed. The more you occupy yourself, the easier it will get.
Obviously this doesn’t work for everything but this is what has always worked for me.