The duty of care thing gets thrown around a lot.
Your duty of care is not to assess the situation for yourself and allow someone to stand when the sign is on! Your duty of care is to inform the offending person until they either comply or refuse, at which point you inform the captain - who will either switch the sign off (after making an informed decision) or intervene with a PA or police (however he/she deems necessary).
Sorry to sound militant, but it annoys me - the 'resolute' attitude some display is the stuff accidents/incidents are made of - and it's not you that is responsible should someone be injured on your flight because you failed your so called 'duty of care' - its the poor bloke behind the iron curtain who knows no better. COMMUNICATE folks!
Sure there may have been announcements, signs, and individual verbal instructions - but that does not mean the person has understood, or worse yet, will comply. Do your job, and inform the skipper if they don't do what they're told.
As for the odd breach you see - well having had a concussion and bleeding from severe turbulence myself - not me! And anyone I fly with gets the full story if they don't give the sign and the captain the respect it deserves.