DanHollis solution is good, but it merely disables the button in software. I'll tell you how to completely disable the button.
You will need:
- Center hole punch. 1/8" recommended.
- A hammer, preferably light-weight so you have very fine control of your swing.
Align the CHP over the power button. The 'Center' in Center Hole Punch is instructive - you need to make sure you're exactly over the switch itself. Done correctly, this will remove the switch without affecting any other components in your MacBook.
Swing the hammer. Sounds simple, but it's not. Make the swing fast and accurate, but don't put much force behind it. The point here is to strike quickly enough to punch out the switch itself, but not with enough force to penetrate deeper and actually damage something. Done right, the CHP will strike through and permanently disable the switch. Too fast or forceful and you'll break the computer. Too slow and you'll just turn the computer off.
Your power button is now completely disabled. If the need arises, you can start the computer by manually shorting the contacts the switch connects to. On my desktop I can do this by arcing the pins with a knife, but the MacBook won't offer you the internal space to do that. I leave this as an exercise to the reader.
You might want to practice your hammer swings before doing this for real. I suggest practicing on a meaty part of your leg. The objective is to swing as fast as possible without feeling pain. If you swing too hard, it'll hurt. Too soft, and you'll just be tapping your leg. However, if the hammer bounces back and it doesn't hurt, you're doing it right. Just practice until you're confident and you'll do fine.