Fly Deterrent?

I am having a party for my 2 year old son and the flies where I live are terrible. I will serve food in a sunroom, but it will mainly be an outdoor party. As it will be about or above 30c I am looking for something that I can use to deter flies. I have Aeroguard but is there anything I can use in trees, the shed roof, or on the ground etc that may deter the flies from even being in the area. Any suggestions would be appreciated.