Same way you kalk aptasia. Turn off your flow, make a kalk paste thin enough to squirt thru a large syringe or small basting-type instrument. Leave on for an hour, before you turn the flow back on. (Hint: Feed all your corals too, while the flow is off.) By tomorrow, they should be dead enough to peel off with tweezers. You'll have to do this again to kill all of it or they will come back.