You can buy Fly trap bags at Walmart. You fill part way with water and hang around the kennel. The only thing that I would suggest is that you do not hang these near the house as they smell like rotten meat. Also keep the kennels clean and if you have gravel or dirt floors keep Lime down. If they are are concrete spray with bleach water solution. And you can spray the gravel or dirt with the bleach solution too. But the bags help alot.

Fly predators will do the trick. You order according to how many animals you have on a monthly basis. They work. The name of the company is Spalding labrotories.

Bayer Complete Insect on the dogs and Purple Python Cattle tags on their collars repel all kinds of insects including flies.

Fly Predators best thing I have used.

Thanks everyone, I never had a problem till last year