I've done some reading online regarding these inverts and found that the only real natural predators are possibly the Copperband Butterfly fish and hermit crabs(article didn't mention the type).  I have at least 2 different types of vermetid snails.  The small ones that are less 1" long and about as big around as a mechanical pencil lead.  I don't mind these, but I'd love to find something that will eat the larger ones that I have.  The larger ones are about as big around as a pencil and as long as 1.5".  I personally think they're ugly and they're mucus webs seem to irritate my SPS at times.  I suppose as a last resort there's always manual removal, but they're so prolific I'm not looking forward to that route.