What are your ways to stop thinking about someone?

More specifically, about someone you really like but know that you won't ever be together with them.This is my situation now - I really like a guy who ignores me pretty often, and even if I know there's no chance for me to start a relationship with them, I still think about them.Sure, some activities gets my mind off them for a while, but this doesn't really work for me cause I actually manage to think about them while doing said activities, for example if I'm hanging out with my friends, I imagine how it would be if he would be with us, etc.Hope you understood me, and I'd love to hear your ways on this case. Maybe that will help me too.