Just a few ideas- though probably controversial !
Could the shop doors be automatically closed and locked if a shoplifter is seen on camera taking things? They would not be allowed to leave until they had returned everything they had stolen. It would only take a few minutes and potential customers waiting inside/ outside would be informed of the reason for the delay.
Also local shops should have photos of the known shoplifters and they should be refused entry into the shops by the security guards.
I think the idea of having to show your receipt before leaving the shop would deter some shop lifters.
Also the security guards should be given a bonus every time they apprehend a shoplifter. Otherwise where is the incentive, as they get paid the same whether they stop a shoplifter or not?
Finally, as Witzend says until the police take real decisive action and the shoplifters are punished there is no real deterrent.