Berghia nudibranchs eat only aptasia. They aren't cheap to acquire. $15 apiece plus shipping. And there's always the possibility that they won't live long enough to do the whole job, before they get sucked up in a powerhead or overflow, or eaten themselves. Their effectiveness in my mind depends on your individual settup and it's inhabitants. They have also been known to multiply in some systems. If you're successful with them you should think about finding someone who needs them before they starve to death.
Tons of other ways to get rid of them as has been said here before. Fwiw, I found joe's juice too liquid in consistency. After some trial and error, I've gotten pretty good at making a true paste of kalk, so that it doesn't fly around the tank at all, and a small dab will smother the small ones that are too small for any syringe technique. Peppermints have never failed me either, but as is the case with everything in this crazy hobby they have pluses and minuses too. Peppermints make real good sushi for a good number of reef fish, and therefore may not live long enough to do the job. As peppermints get bigger, so does their appetite.......for things like snails and zoa's and polyps. If you have the capacity, you can set up a small tank (5 gal) to house only peppermints, and place one affected rock in their tank at a time. I'd recommend applying a true kalk paste, and a good dose of persistance on your part. hth