There are a couple ways. Easiest way is if the rock (or rocks) are easy to get to, pull them out and torch them (as pharmer said), or you can use gel superglue and goop it all over the hole in the rock they're in.
second option (but not 100%) is try to get a few peppermint shrimp and see if they eat them.
Another way is to get a copperband butterfly, or aiptasia eating file fish and see if they take care of them.
You can also use a high powered laser. Those fry them.
Aiptasia-x is another solution.