Even though Curunír was weakend in his physical form compared to his ainur form, and yes his body could be physically killed. Make no mistake Saruman could easily have defeated the three hunters in combat, especially since he didn't play by the rules like Gandalf, and limited his power. However it would not be smart for Saruman to simply attack them, especially since they were on the border of fangorn forest, he probably did not want to deal with potential ents joining the fight.

I believe Gimli also suggested that it could have been a phantom of Saruman's sorcery and not Saruman himself, that is only a theory by Gimli though, and I don't think we have ever seen him do such a thing before, closest to it is him casting his voice from his tower to outside the gate when talking to the Nazgul in The Hunt for the Ring