Simply put, no, you cannot say "Warden kills only."
I had this discussion with Travis a while back, so it may no longer be consider as sound logic, or proof of anything. No idea. I was told this. I do not agree with it.
The function of the guards is to have the round proceed by killing the REDs (though logically, this only means rebels). Restricting killing entirely from the guards is defeating the purpose of their existence. However, a warden can say something along the following lines:
"For the remainder of this game, I'd like the guards to try and be more lenient."
or
"I'd prefer if the guards were to consult with me before killing REDs."
You cannot restrict the killing of rebels. You can somewhat restrict the killing of non-rebelling REDs.