Have you heard of the fish called aiptasia x? :ape:
But really that'll probably work or something like joe's juice, which I've never used but probably does the job too. Just make sure it doesn't touch your corals. Quickly siphon it when it's killed. Unless it's between corals like zoanthids then forget about it, it'll kill the corals too...
Or play it safe, I read a thread that someone used an underwater epoxy and jam it down or use glue. I can see that happening.