There is a wonderful gizmo called the "crowbar circuit". You put a fat Diac across the PSU output and a zenner diode to trigger it should the volts rise above your max threshold.

When you trigger a Diac it stays triggered, hard on, until the Volts go away. A cheap Diac can handle lots of Amps.

In this case the Volts go away because the fuse blows. You then replace a 10p fuse rather than expensive hardware.
It's called a crowbar because the effect is like dropping a crowbar across the power input terminals