If a crewmember gives you instructions, that's your rule du jour and unfortunately, the remedy is after your flight has completed. The last thing anyone needs is a crew member unjustifiably putting something on your "permanent record" which can include detention, arrest, et al., for the most innocuous thing. And most likely, the cockpit will back the FA up while you cause a flight delay and are awaiting the authorities ... even if it isn't you who caused the delay and did nothing wrong. I expect that the same crewmember has never put her food on the floor and quite possibly had some in the overhead bin (or perhaps a closet) as she spoke.