"...all we can do is inform them of the risks and that they are no longer insured should the aircraft hit unexpected turbulence. I realise this poses a risk to those of you who do adhere to the seatbelt sign but we cannot physically restrain a customer who refuses to take their seat..."
Just to add that saying this to passengers is very dangerous. You are entering into a grey area. I would strongly suggest avoiding insurance discussions.
If the passenger sues the airline, the pax could say that you didn't make it clear engough that you were possibly entering an area of severe turbulence.
"Please sit down sir, the seat belt sign is on"
If you continue to repeat this, then there is no grey area.
Then all you have done is instructed the passenger to sit down. By not doing so, he is disobeying the lawful instructions of Cabin Crew, and is therefore commiting an offence. He will have no leg to stand on.