How will the shear be operated? ie, foot pedal, hand lever, automated, etc...? is it operated by highly skilled machinists, or general laborers? Is poduction rate of this operation important?
All these things will point you toward a solution.
The "pure" solution is to prevent the machine from moving while any body part is in it's path; or you can think about it as making sure no body parts can be in the machine's path during it's operation.
For a simple solution, maybe consider vertical plates parallel to the shear on either side of the shear with thin openings for the rebar, big enough for rebar, but to small and/or too far away from the shear for body parts.
To stop the kick-back you need to mount a large plate to help peal the rebar from the sheer. I agree with sbd2 in the rest of the guard. I am asumming you are using general labor for this operation.
