Yes, I did miss the part about destroying her fence, present them with a bill for repairing the fencing, take pictures of the way posts are bent showing that they were bent from his side of the fence, but that would be considered a civil matter. Do you know were your property line is and if the lots share the same lot line, here there is about 3 feet between lots for utility easements, that might be worth checking out, and revoke your permission of him using your fence line.
But depending on zoning in that area, there maybe nothing that can be done. Some rural areas don't have alot of zoning regulations in place. That's were talking to your county manager, or planning commission, comes into play if you want the regulations changed, but be aware that they may have grandfather clauses in the changes.
Flies are a problem wherever livestock are, most of the people where I live have livestock, the flies are unbearable in the summer here too. I should own stock in Raid.... You can buy fly predators that help alot to keep the flies down around your place even if they come from next door.