What's the easiest way to prevent a voltage input getting higher than a fixed value?
A crowbar circuit is one way- it's (essentially) a Zener plus a thyristor. Once the Zener voltage is exceeded enough current flows to the gate to turn it in, and it "shorts" the power supply blowing the fuse (or depending on inherent current limit of the supply). The Wikipedia entry shows a nice circuit that uses the synthetic Zener TL431 chip and a triac, which can have excellent unadjusted trigger point accuracy.



A power supply from a reputable supplier will have over voltage protection of some kind (OVP) built in, most likely. Using a garbagey one on an expensive laptop would be inadvisable, in my opinion. Personally, I use the OEM power supply and an inverter, which gives a lot more protection at the cost of some inefficiency.