Black fly problem

[rowenj]

I have a potted cactus that i have been keeping on the windowsil. Now it's plagued by these small black fly, they seem to live in the soil and lay their eggs there and are a nusance. They fly all over the room, I often see one or two on my monitor screen.
My question is how do I get rid of them once and for all?
So far I have resorted to fly spray but this does not kill the eggs/lavae or some just survive.

[douglas9401]

Repot the plant and dispose of the old medium. Check the plant thoroughly befor replanting.