There are programs that can intercept the output of the keyboard and send a different signal to the computer. They're called Keyboard Macro Recorders or Macro Utilities. There are many free macro programs.
The idea is that you record a sequence of keystrokes, just like giving your phone a telephone number to remember, and then you assign a key (hotkey) to that recording. Then whenever you press that hotkey, that sequence of keystrokes is given to the computer instead of the original signal sent by that key.
So theoretically a macro program could let you use the Power Off key as the hotkey, and Delete as the sequence of keystrokes that are sent whenever you press it. But they usually have some restrictions on which keys can be used as hotkeys. It might be hard to find one that will let you use the Power Off key as a hotkey.
The Delete key on a keypad is usually a Delete key only when NumLock is off. When NumLock is on, it's a decimal point. Do you have another keypad key for the decimal point? If so, you could yank out that PowerOff key with a pair of needle-nose pliers and cover the hole with a piece of plastic and some epoxy.
Bear in mind that only a truly evil manufacturer, worse than Microsoft, would put a key like that on a keyboard. What you really should do is burn the keyboard in an air-tight container, because it's probably full of anthrax or something.