I already know that one of my players is likely going to attack Baba Yaga as soon as he can. I'm planning on allowing that to happen...if he dies, he dies. If something unexpected happens...then that's what good stories come from.
If you want to actually stop him, I think the geas idea is a good one. You could also have her physically rip the mantle away from any player that attacked her, stunning them for a round while she teleports away.
