I agree with Mr 'QSK'.
In my experience working at several / various FSC's & FSU's, we were all acutely aware of our responsibilities, particularly what we 'could say/do' and what we could NOT.
And 'tearing up' a FPL was NOT on the agenda.
If submitted, the FPL would be put into the system, as the pilot may decide to go and 'have a look' - later, or seek other advice - instructor etc.
A little green smiley face often was an acknowledgement of a well prepared and legible FPL, and all available assistance was the attitude of the stations I worked at. No flashing blue lights here mate.....
As has been stated, any particularly 'stroppy' situations would be referred to the Senior Ops Controller (SOC) in those days, who DID have the authority to direct the pilot accordingly.
I do not recall ever having to do this as most advice given was in a professional manner, stuck to the facts / requirements of the situation, and was accepted accordingly. What the pilot did later was up to him /her.
No Cheers
