You don't want a you-vs-them situation for sure.If they play at the wrong time, give them a 'dagger' glance.For playing too loud, do a warmup and have the lowest instruments play, add in middle instruments, then upper instruments. The loudness follows that same pyramid. If they play too loud during that, give them the hand and quiet them down. "The hand" looks kind of like this...
I never had the misfortune to have a praise and worship group in the churches for which I was the music director.
That said... remember that you are the boss. .
I wouldn't be surprised if the drummer is oblivious to it or thinks they should play really loud like a rock band.