Easy way to stop this without messing with any electrical: Find another seat belt (Either form a junk yard or see if they sell replacements anywhere online for cheap) cut the belt off so you only have the part that clasps in the buckle. "Plug in" the seatbelt clasp and leave it in there. Your car will think you have the seatbelt buckled all the time.

 

One quick and easy way to "kill it" would be to get in a wreck and let your head smash through the dash and short out the indicator.
(Oh, and be sure you leave a note in your car for the coroner to let us know if that took care ofit for you.)