The DNSP/Supply Authority has a variety of methods available to it to force systems offline in the event of a failure.
Most small generators require a manual disconnect switch, so at a minimum the utility could send a worker to pull them out of service. For large sources, you could simply install a dedicated phone line to have an operator manually shut down the generator. In either case, the manual reaction time is likely to be too long.
Through the use of signals sent though the power lines, or on dedicated comms line. Power line carrier communications could be installed in all inverters, checking for signals from the utility and disconnecting either on command, or if the signal disappears. Such a system would be reliable, but expensive.
To deliberately force a section of the grid into a condition that will guarantee the inverters disconnect.
An example is a large bank of capacitors that are added to a feeder, left charged up and normally disconnected. In the event of a failure, the capacitors are switched into the feeder by the utility. The capacitors can only supply current for a short period, ensuring that the start or end of the pulse they deliver will cause enough of a change to trip the inverters.
There is no NDZ for this method of anti-islanding protection. The main disadvantage is cost, the capacitor bank needs enough capacitance to cause changes in voltage that will be detected, this is a function of the amount of load on the circuit. Large banks would be needed, cost prohibitive on the part of the utility.
Anti-islanding protection can be implemented through the use of the Supervisory Control and Data Acquisition (SCADA) systems already used in the utility. An alarm could sound if the SCADA system detects power on a line where a failure is known to be in progress. This does not affect the anti-islanding systems, but could allow any of the systems to be quickly implemented.