The Helmet of Charm Protection is cheating, for those who don't know, since it doesn't even exist in the game unless you hack it in. I've never heard of a Charm Resistance spell, but a Potion of Clarity is probably what you want. Everything that Midget Gems suggested also works. What I do is give everyone ranged weapons, and take each Sirine one by one, hitting her constantly so she can't get any spells off. And if somebody does happen to get Charmed, I just call Algernon's Cloak into play and Charm that wayward soul right back!
NEVER let your best Fighter get Charmed if he's wearing the Boots of Speed!!!