At the league's next night out, the first round, or in crueler constitutions the entire tab, lands on the loser. Consumption mysteriously spikes that evening. Simple, social, and self-limiting only by the league's collective conscience, which is to say not at all.
Budget note: this is one of the expensive ones. Decide before the season whether the loser pays, the pot pays, or the league splits it, and write that down. Money arguments kill leagues faster than collusion.
Seen in the wild: trophysmack.com