If they were Summit or Ascent members, they could be moved up free of charge. If they were poachers, the fix is the FA being diligent and telling then to get back to their assigned seat or come up with
the credit card machine and ask them to pay for their seat. No one would think of taking snacks off their tray without paying for it!! But it's the FA job, they do know who Summit, Ascent and Classic Plus passengers are.