Under what circumstances can a flight attendant unbuckle and get up during landing? Lately I've been on several flights where the flight attendants had to talk sternly to passengers that were getting up either just before landing or just after (which is quite obvious to everyone since the talking to is done over the PA system).

Now this made me wonder what FAs can do if the passenger just refuses to comply? Can FAs get up and forcefully seat passengers (would they have to talk to the pilot first to abort the landing)? If not, are there any circumstances where the FAs can/should/must unbuckle during a landing?
An FA is a member of the crew and can get up or sit down as they wish during any phase of the flight. As to what they can do, they can have the pax arrested the moment the aircraft door opens. If that is not a good reason to obey instructions, I don't know what is! If the pax becomes a threat to the safety of the flight, yes, they can certainly get physical to put an end to the situation and secure the cabin.