A natural fly spray will be effective but not against every types of flies unfortunately.
You can try this one that works pretty well on my horse:
1 cup Water
1 cup Avon Skin So Soft Bath Oil
2 cups vinegar
1 tbs. Eucalyptus oil
I recently find this one that I'll probably try:
3 caps of dettol from a 500 ml size bottle
250ml Avon Skin so Soft and Fresh.
1 teaspoon each of
cedarwood atlas oil