I've had vermetid snails kill 2 colonies of zoa's.
Here are some links that coroborate their negative impact.
http://eco.confex.com/eco/2007/techprogram/P7313.HTM
http://www.zoaid.com/gallery2/main.php?g2_itemId=536
http://www.hopsreef.com/vermetidinfo.htm
I am willing to try the mantis shrimp, but I want to make sure that the people suggesting that know what vermetid snails are. They aren't mobile snails like most mantis prey. They attach to rock and are stationary. The only videos of mantis behavior I've seen make me think of them as oportunistic predators, not rock scraping scavengers. Your thoughts?