I believe that as with everything they will wax and wane to the available nutrient (food) in the tank. First thing would be to evaluate the amount of detritus they have to eat and try to - in effect - starve them done to some level. I believe that the six-line wrasse may also be a natural predator, but do some research on that to be sure.
If you can put up with them they are a good clean up critter though.....