problem with black fly

[dave]

hi everyone. i've a plant in work that has a considerable number of black
fly (assuming that's what they're called) that live in/on the soil. does
anybody have any idea how i can get rid of them?
the plant is similar to a swiss cheese plant and lives in a 4ft x 1ft x 1ft
trough. the flies are becoming a problem.
the flies themselves are virtually identical to green fly only coloured
black. i've tried spraying the plant and soil with soapy water but this has
made no difference.

any help or advice greatly appreciated.

cheers, dave