Broncos LB Von Miller -- The Unwritten Peyton Manning Rule

Peyton Manning is OFF-LIMITS in practice ... and any defensive teammate who even thinks about SNEEZING too hard around PM could wind up on a missing persons list ... this according to Von Miller.

The Denver Broncos superstar linebacker -- who signed a $21 million contract in 2011 -- was at the Playboy mansion this week ... when he spelled it out for us -- NO ONE TOUCHES MANNING.

"I have never even touched ... even got CLOSE to Peyton Manning," Miller said ... "We know how important he is to our team. We don't get close to him."

Of course, Manning's not just valuable -- he's fragile -- with serious neck issues.