well have they layed their eggs yet?
if not give them something smooth to lay their eggs on like a clay pot, then you can just remove the pot, but make sure the eggs are not exposed to air when removing them from the tank.
if they have layed their eggs you can try removing the rock they are one is about the only way i would know OR you can let they eggs hatch in the tank, and when they start to hatch make sure the light is off in the tank (i believe clownfish hatch at evening/night anyways) and get a flashlight and shine it in a corner of the tank and they fry will swim towards the light and you can collect your babies!
mabey someone can give you a more detailed way to do it. what kind of rearing tank do you have? what do you plan to feed the babies?