Reducing glassware size seems like a good way to address the issue. If the guy is drinking 5 servings of 24 oz. pours, he should probably cut back anyway.
I set out a bunch of taster glasses for friends. If they want to keep refilling them, that's fine, but if they're also coming over for "a visit" then hopefully they're not going to spend all their time disappearing into the back room for a refill. I also like the idea of an ingredient fund, though I haven't needed to go this far yet.