The only thing one can do is advise the people in the back that the seatbelt sign is on and for their own safety and the safety of others they should sit with their seatbelt fastened. They are not being held prisoner, they are not in forceable confinement. If they choose to disregard the advice from the Cabin Crew and ignore the seatbelt sign, there is not too much anyone can do...
If you have advised them of the sign, you have done your job and the onus is on them if they choose to disregard the sign.
