Is this an unreasonable request?

You can't really control who your friends hang out with & it's a bit selfish to try to persuade them, but I see where you're coming from. It obviously makes you pretty uncomfortable & you should try to further explain it to your friend (Ben) if you haven't. The most rational thing you can do though is just try to put it out of your mind & not let him get you down anymore. You're obviously doing everything you can to keep him out of your life, and you don't need thoughts of him creeping back, that's just unnecessary negative energy. Keep doing what you do but try to let go a little - of the feelings of resentment. You don't have to forget, but forgive & you'll be more at peace with all of this. Good luck :)