It's quite simple.
Being drunk on an aircraft is an offence, prosecute the offenders.
Then investigate how they were on the aircraft, drunk.
If the gate staff let them through, drunk, pull their airside passes.
If the CC let them on drunk or they became drunk in the air, pull the #1's (for not controlling the cabin) and the captain's airside passes, as s/he has ultimate accountability for the conduct of the flight.
Of course, if the pax drank their own duty free and refused to hand it over, then no punishment for the CC and extra punishment for the drunken pax.
If drastic action was take, word would soon get around and this behaviour would be stamped out.