A word of caution bird wrasse are NOT considered reef safe so this is not an option for everyone so use with caution. In my opinion introducing a fish that is not reef safe into a reef to get rid of something that may or may not be killing your snails is not a good option.
First before jumping to conclusions on a cause can we see pictures of the Whelk snails? To be honest I have been in this hobby for a few decades and I have never came across these so they are not very common in the aquarium trade. Or I would have had them or met others that legitimately had them at one time or another. I have seen people that had a whole lot of problems in their tank going on and they didn't want to admit that it was something they were doing causing their problems. So they felt more comfortable to blame it on whelks when it was not.
Not ideal water quality, lack of the right food source, something toxic in the water or fish\ Shrimp are the more likely and common causes of snail death. With that being said it can't hurt to be cautious and ID said Whelk snails as this is always good practices to be cautious. If you do have them manual removal is best, don't introduce something that in the end could end up causing other problems for you. You can set up some traps with food in them to entice them into the trap for removal. It is a pain but in time the numbers will dwindle.
