When I was CC back in the 80s, we always used to tell unsecured pax that 'they weren't insured' while they were walking around/going to the loo/getting things out of the overhead lockers while the FSB signs were on.
I am not sure whether that was actually (legally) the case but it usually got them to sit down and strap in - for a minute or two anyway!
I suppose that if a passenger were to be badly injured during turbulence, the amount of compensation/damages they might be awarded could be reduced or refused if they had contributed to the damage themselves by ignoring the signs and requests from CC to strap in.