C.
We had that TERRIBLE problem...I mean thousands....so gross...
We got some fly tape. It's this sticky tape that you put around the area, and the flies area ttracted to it and then stick to it. It's so disgusting but it worked...I thought it was just making things worse because it was ATTRACTING them all to it, but once they were all caught, they did go away.
We also did something....I don't remember exactly what...but it involved a funnel for the flies to go in and then not come out. The fly tape worked the best though. good luck