Its a real problem.

Its either no smoking, which I agree with, or I try firstly to see what happens if people who want to smoke, go and have a smoke at the far end of the room. They can at least still hear the music. The room is big enough to at least give it a try.
If that doesn't work, then it will be a no smoking venue.

At least I will have tried to accomodate the smokers.