A death spell is not a wise idea. Plus it most likely wouldn't work anyway but you can still suffer from karma for casting a spell like that even when it doesn't work. So not a good idea.
Your best bet is to do a spell to remove the person from your life if you can't get them to leave you alone without magick. You always have the option to remove someone from your life without magick even if you don't think you can. It may involve having to get outside help, but you can be rid of them. However, even with the non-magickal methods a banishment spell of the person will aid in making it easier. As with all spells it will be better if you make the spell yourself. Look at some that are already online to get some ideas but don't forget to use common sense when looking at them. Then study up on the basics of spell casting. You want to know as much as possible about how and why spells work, understanding the elements and color correspondences, how to cast a circle, and etc. These are all important things to know before ever trying a spell.