If someone tells you, “I am friends with all my exs,” good for them. If you are now their ex and don’t want to be friends, there is nothing wrong with that, and don’t let anyone tell you otherwise. It is up to you who you have in your life. And if that person is toxic, or someone you just don’t want to keep up with, that is a good reason. It could also be someone’s way of lingering and keeping tabs on you.