[Mike Simonds]

I have encountered a few coaches that teach their players to contact the QB on every play whether he has the ball or not.

The referee is primarily responsible for protecting the QB on running and passing downs.

It is correct when the other posters state that the QB is fair game to a legal block or some other legal contact by the defense.

However, if it is obvious to the the defense that the QB no longer has the ball, then the covering official must call a foul for any illegal contact on the QB. This can take the form of an illegal block in the back, clip, block below the waist, etc. If it falls under the criteria for a personal foul then you can call that also.

In summary its a judgement decision.