Mineral Blocks and food supplements for flys are a waste of your time, fly sprays, head nets, environmental control (frequent mucking, fly traps, blankets etc) are your best options. I hate flys, and despite your best efforts they will be a problem.
Some great natural fly traps can be easily made at home (although they smell)
Use a pickle jar, put some meat and water inside it not too much and then close the lid, pop a small hole or to through the lid (enough for a fly to enter) and place them around the barn. Flys get in but can't get out. It's disgusting but in a week or two those jars will be filled with dead flies and then you can do it again.
