what do you say to people who just come to sing and not buying anything

[Lonman]

Personally what I THINK is they should be spending money in a bar - you don't necessarily have to drink alcohol, if you don't drink then soda, food, coffee other types of virgin drinks (unless it's a tavern). THEN the bar needs to CHARGE for EVERY drink coffee, soda, water included - no free refills (UNLESS they have been spending money all night & need to try to sober up a bit before driving out).
Again, you really can't say anything, you can suggest it to the manager/owner, & they can deal with the non-spenders accordingly.