How do I get rid of these pesky Flies?

There are two areas around my home where flies and gnats congregate everyday. There's no food or plants to attract them and I don't understand why they're there eachday on the back and front porches. How do I get rid of them and keep them from coming back. I'm thoroughly annoyed, and recently they caused a maggot infestation in the bedroom trash, uugh. They have to go!!!