Tell them to stop, assertively. It’s a fad in pop culture right now and these girls have taken it to far. Actually, the vampire obsession is a common thing in the teen world currently. I guarantee it will die out sooner or later…and then the next hyped “thing of hotness” will come. Really, just make it clear that you won’t put up with it. Get a teacher’s backing if necessary. And remember, this is a passing thing.