I totally agree with getoutofmygalley. We ask for pax full attention in the demo spiel. If they continue talking I will pause safety demo and ask for silence in the cabin as there may be people who have not flown before. If this doesn't work I will pause one more time and say that it is the last time I will ask for silence before we will return to the gate. The crew also individually ask pax to be quiet. A lot of the time it is people who have flown heaps and reckon they know everything so I also go and have a quiet word at the end. Other pax always get off saying good on you for shutting up those twerps. Footy groups we usually go and say to them 'we know you want to have a good time onboard but now is the serious side of the flight.....'
And as previously said if you can't get their attention for this, what chance is there for them to do anything else you ask or say....