How do I respond to someone down the bar buying a drink for me, but I’m not interested?
Bars shouldn’t let people buy drinks for strangers and shouldn’t facilitate this very outdated—and quite frankly predatory—practice. This might be extreme, but if out of nowhere a bartender were to bring me a drink paid for by someone else, I would refuse it, and then leave. However, if you’re feeling like buying a drink for someone and don’t want to feel like a creep, you should do the same thing that everyone should do in every situation where they don’t want to be a creep: Get consent. If it seems like someone else would be interested in you buying them a drink, ask them! And be gracious if they say no in response. – Bon Appetit
Real Life of an MSW 