It is not permissible to transmit "you're on guard" on 121.50, but it is permissible to transmit a false message stating that you are D&D.
I think what we are saying is that "you are on guard" is often heard by the perpetrator as "go ahead".
Basically this is all down to poor RT discipline. My own pet hate is the AMOUNT of aircraft that start their call to me with " and Callsign....." which can sound like "PAN callsign"
I did start off by politely asking they don't - now I just acknowledge their PAN call and ask the nature of their emergency. Slowly the message is getting through!
Fred