I feel for you guys... we used to deal with all the time at Milkwood Farm... we tried lots of different policies and none of them really worked... just like in the US of A drinking happens alot in Australia.
To put us into context we our farm had an average population of about 12, but with regualr coruses hosting up to 40 extras for up to 2 weeks at a time. Courses were generally alcohol free, but we didn't "CSI" empty bottles if we found them.
We tried banning it, but the cost was just too high.... a few people sharing a drink in the evening can be a great social experience. We also had a weeknights ban (to try and reduce instances of alcoholism) but I felt like a school teache enforcing that..
Eventually we just let it be known to the crew that intoxication was unacceptable and dealt with it on a case by case basis... that worked best... but was still hard work.