How to reject a person nicely? (Shenzhen)

Basically I don't have any particular scenario in mind but I'm interested in finding out people's different ways in solving the same situation. Yes, as titled, suppose it was in a bar, or somewhere you have to face the person, what would you guys do or say to a person who you know s/he's fancy you but you want to mean 'no' without hurting him/her?