It is sad but true that pax always think they know better than the cabin crew, but the reality of it is that we will never have the full attention of pax during the dafety demo. Even the emergency demo, pax will be panicking, thinking " am I going to die" not listening in the slightest to the brief. This will always be a touchy subject , and all we can do is to do our best and hope that at least 1 in 3 have listened and will be able to help others in an emergency.
I think the cabin crew have every right to move or even off load pax if they are blatenly ovious in not paying attention during the demo, especialy when seated next to an exit and already having a warning from the vrew member concerned. After all, there are air navigation orders which do state the 'rules' and the consequences facing the pax if they fail to comply.
When a pax buys a ticket, they are buying into a legaly binding contract of terma and conditions in which the airline operates by, and like any other kind of contract, there are consequences if broken.