That's tough. Seems like if they're going to a tiny and consistent group of friends' homes in the evening to have a drink, then they're effectively an extension

That's tough. Seems like if they're going to a tiny and consistent group of friends' homes in the evening to have a drink, then they're effectively an extension of your family and any spread would be limited to that relatively small group (unlike in a bar). And if they're actually going to a bar instead, then just close the bar at 6pm. A general curfew seems like a chainsaw approach, instead of a scalpel.