We've done a fair amount of playing for events sponsored by historical societies and re-enactment groups over the years. Many of their members are excellent trad singers in their own right. They are welcome to come up and sing any time they want to.

Drunk yahoos in bars are another matter. Just say, "No!". Do not let them sing one song because it will only encourage them to sing more. And they'll usually have a handful of friends sitting at one table who thinks it's funny while everybody else in the place wishes they'd shut up and they're all getting mad at you for letting the bozo on stage in the first place

And, if they try to touch my guitar, the punishment is commensurate with the value of the guitar I happen to be playing at the time. If they touch the Gibson, they get their fingers slapped. If they touch the Santa Cruz, they get their fingers broken.

Bruce