[Lonman]

We have that problem on occasion. Usually it doesn't turn into a problem, but they do not give any free refills anymore, charge for all drinks - including water. If someone is not drinking they are asked to leave at the bartenders discression.
I have made the suggestion to several in the past & have actually seen it in action at a couple places.
$5 token purchased at the door. It's not a cover charge as this token can be redeemed for $5 in either food or drink. The ones that were planning on spending money don't gripe because they get their $5 back to buy their drinks. The water sippers have at least paid SOMETHING to be able to sit & sing. Yes it may drive them away - but a non-spender is taking up a potential spenders seat, or it may get them to start buying a little more.