Learned this lesson many years ago.You never turn it down because it's business for the bar. If they actually take an order you can just get a soda. If they bring you a drink you say thanks, set it aside, and give it to someone else or just let it rot at the back of the stage.
Never forget why you are there: to sell alcohol. Music is just a means to an end.
We're usually in the middle of the action anyway so it's easy for him to set it off to the side and then carry on and keep cranking out the music. Never had anyone get upset that the libations weren't finished....I'd wager to guess most don't even know or remember.