If the rock is easy to remove from the tank you can just lift it out of the water for about 60 seconds. This is usually more than enough to kill a sponge. If it is a large amount of sponge it will smell terrible once it dies.
If the sponge isn't growing over other corals I would just leave them alone. However, large growth of white sponge can be a sign that not enough water flow is reaching that area.