If you're looking to render your power button useless (and on a PC), you can simply unplug it from the motherboard via the pw_sw (power switch) cable or something similar, keeping in mind you will have to have an alternative method to power your PC on (such as wake-on-lan, or wake via keyboard/mouse found in the bios settings if motherboard supports it).
If you're on a laptop.. maybe you could tape something over the power button?
You have small children about huh?