Lock Axis of an IK constrained Bone

Hello there, I have a problem with correcting the mesh of a rigged character. As you see in the first image, the shoulderplate is not going thru the rest of the mesh. But if I roll the arm with the pole target the shoulderplate is going into the rest of the mesh. I already tried out to add another bone that is parented to the upper arm bone and lock it’s local Y rotation but it seems that it does nothing. Shape Keys aren’t an solution aswell because they deform the mesh. How can I manage to limit the rotation of the shoulderplate?