All you need to do at first is talk to them and explain how they make you feel uncomfortable and how things need to change if you’re going to continue to see each other. If they notice everything you say and they genuinely start to work on giving you space and reducing the severity of their obsession with you, then that’s great. However, if they react badly, threaten you, or beg for you, they’re probably never going to get better, and the best thing you can do is leave them and remove them completely from your life.