Being off the 'old school' I believe that pax. should be accutely aware that for safety etc. etc. the F/A will be obeyed, that the pax. will behave and do as they are instructed on board an aircraft...and this wrong seating situation is just one which is indicative of the poor attitude some pax. unfortunately have...A passenger behaving in this manner at the beginning of a flight could well be a greater problem during the flight...it is certainly an indication
The message to them of appeasement from either the airline, F/A or righful passenger merely allows them to continue to behave in this manner and sets a poor example to others. It is like the school bully who sits at your desk or takes your pen etc. and will continue to do so - why? very simple because he is getting away with it and hasn't been stopped...and where does it stop, air rage next? I used to punch out the school bully even when they were twice my size and they never came near me again
An F/A should not be concerned having to ask a pax. to move nor should the cabin crew be placed under any pressure by their airline not to delay an aircraft where a case of cabin safety/discipline is in question