I use Kalkwasser. You can get a small tub of it at the LFS pretty cheap. I mix it up into a thick paste, shut all the flow off in the tank, then use a syringe less the steel tip and squirt some right in the center of it. When it closes up I cover the rest of it with more paste.
Being so close to the zoas you'll probably kill a few of them as well, but they will grow back in time.