Gnats on compost

My son and his wife have become vegans. They live in a city, started a small garden, and bought a composter. He says he now has swarms of gnats in and around the composter and wants them gone. Yeah, I know, but he doesn't consider insects animals to be protected...

Does anyone know of an "organic" method he can use to eliminate the gnats and still use the finished compost on his garden? I guess this will expand to other insects, etc. as his garden grows so any successful methods of pest control will be helpful.

Personally, I've used chickens, ducks and an occasional quick pesticide spray when needed. His wife doesn't want chickens although they are allowed in his city.

Googling "gnats around composter" got several suggestions. One is to hit the compost pile with a big dose of Bacillus thuringiensis, which attacks the gnat larvae. Another suggested a heavy infusion of coffee grounds. Additionally, their compost is likely too wet, and they're leaving material like fruit remnants on the surface of the pile. Adding in dead leaves will dry the pile out a bit, and keeping wet waste like fruit and grass to a minimum and always inside the pile will help reduce insects.

Thanks, I will give him this information.