Android - How to prevent someone from powering off an android device?

You cannot (barring any hardware tweaks.)

If you root your phone and install a custom ROM, you may be able to remove the "Turn off" and "Reboot" options from the Power menu. However, Nexus 5 (as do most modern phones) has a hard-wired feature, where holding the power button for 10 seconds shuts the phone off. This is done so that you can restart the phone if it freezes up. Since Nexus 5 doesn't have a removable battery, this hard reset would be the only way to shut it off without having to wait for the battery to drain.

If you are technically inclined, you can always make hardware modifications to the phone which would prevent the Power button from functioning normally. However, such mods are not for the novices, and will certainly void your warranty.

You could make/modify a phone case so that the power button is covered. This would avoid having to void the warranty.