The power company should send an announcement to all paying customers that on a specified day and time they are to disconnnect from the grid for 5 minutes. When the time comes, the power company should up the voltage going through their lines by 50%. That will fry every appliance still attached to the grid. If the power thieves have no more appliances, they won't need to steal the juice.
Good plan, but no plan is perfect, and many of the paying customers would also not get the message. A 50% increase may not be enough either, lots of equipment could be built to handle this.
A better plan would be to put regulators on all paying customers lines and then you could do your high voltage burn on the illegals, but the regulators are expensive and it may be easier to just pay some other poor people to cut the wires.