my feeder sudenly has problems with black flys. do they keep hummers away? any suggestions

I can't say that I have every had a problem with flies taking over a feeder. I do have one land on a feeder every now and again but they don't stick around nor have they ever deterred hummers from using the feeder.
What type of feeder are you using?

Penny
NY
USDA hardiness zone 6
Heat zone 4
Sunset zone 39

using platform feeder and bottle one with bee guards

Rosiedc
I don't think they will keep the hummers away from the feeders. I have seen hummers chase bees and vice versa and then the hummers come back to the feeder. So they will probably do the same with the flies if they are bothered by them being around.

Penny
NY
USDA hardiness zone 6
Heat zone 4
Sunset zone 39