Having refueled many aircraft (choppers included) with a nozzle, the responsibility for not over filling rested with me. Even if the pilot monitoring, tells pilot liasing to stop, then liason says stop, the elapsed time will be about 1 second before flow is stopped. Thats to late to stop a spill.
Another factor is that when using a refueling gun, all sources of ignition should be removed, that means aircraft power, so no ability to monitor fuel level on the gauges.
Its not rocket science to look and listen to the fuel going into the tank, and is a simple matter to ease up on the trigger to slow the feed.