No matter how clean anything is, flies will be there. Most barn flies eat blood, they need damp areas to lay eggs and hatch the maggots. Heck, flies get into spotless houses. They only eat a tiny amount of stuff, so no one can alleviate every fly. Just do the best you can.
Sticky fly tape, spray, fly swatters and my favorite- a shop vac at night! They are easy to suck up if its cool and dark!
Blue light fly zapper. We have hell with flies. Only problem is we kill innocents. Another thing I use is a fly trap bag. It has a chemical that when activated stinks like rotting flesh. They get in but cannot get out
Yes, I use the fly trap bag with the stinky yuk in it. Thousands get in it. The only good thing about Winter, here is that all the flies go away. (then its try and rid the barn of all the cobwebs, that is neverending).