I've had this problem before too, although the "over drinkers" are the ones who usually help me out on brew day. I think it's easy for people who aren't familiar with homebrewing to assume that the beer just materializes without investment or that it's just a free bi-product of our hobby. I have no problem laying down the law when it comes to giant glasses. I brewed a pumpkin beer every year for a Halloween party. The first few years I was just excited for people to be drinking something I made so I didn't worry about collecting funds. Two years ago I put out a jar at the kegerator with a note that said "Homebrewing isn't free. Please contribute to the cause.". Three hours and 15.5 gallons later, there was .35 cents in the jar. Needless to say, I still brew my pumpkin beer every year, but only a few pints emerge for close friends and family at the party. Although I must say, it can be flattering to have a keg of homebrew go so quickly.