Edited by duckduckturkey - 7/8/11 at 9:44pm
My advice isn't going to help you immediately, but it might come in handy in the future. Frogs.
I had a HUGE ant problem last year. Nightmarish. Hitchcockian. Think, "The Birds" only with nasty, foul, billions of ants. I went out, got a bunch of tubs, filled them with tadpoles and walla! This year, I still have ants, but ha! hardly worth the mention.
Yes, flies are faster than ants, but frogs are faster than flies.
Trouble is, you can't just go out and catch frogs. Frogs hop away looking for home. Tadpoles grow up in their sweet little ponds and think, Aaaahhhh Home! So they stick around when they get to be frogs. Also, you will need a naturalized area for the frogs that the chickens can't reach as I do believe chickens will peck at frogs if not eat them outright. You'll also have to keep an eye out for mosquitoes during the tadpole stage. It's easy to net them out, but you have to be sure you don't get the tadpoles, too. Tadpoles don't eat mosquito larvae. Fish do, but they also eat tadpoles. So you have to wait until the tadpoles have turned into frogs before putting the fish in the ponds. There's a trick to it, but trust me, it can be done! I've done it! And it's actually kind of fun.
Just be sure you don't get bull frogs. They get big enough to go after chicks!
Not much help now, but look to the future!
Don't people make traps with jars and lids? so the fly goes in to get the rotten meat, but then flies up to the light and can't get out? Works like those carnivorous plants.
Which, I've had those and they do work! I don't know where you live, but it's worth looking into. (Again, not a quick fix.) My plants were doing well for a while, but then failed for the following reason: They do NOT like chlorinated water, so, if you don't have total control over the watering situation in your area, they might not be so happy. There are some great books on the subject, check them out, then check out the plants, they are so cool! If the flies are as bad as you say, you will have to make a habit of cleaning out the plants traps from time to time, as over eating isn't any better for the plants than it is for humans. But I prattle. Go! Read on the topic and have fun with it!
In the mean time... have you tried herbs?
I know citronella, and all the other things you mentioned are OUT! Do NOT go there! But what about rosemary? basil? or (gag) corsician mint? I'm asking, not advising. Hops?
Boy, now you have me wondering
Best of luck!