Wear your seatbelt??
There should be a operation listed in the owners manual to disable it.
Here is what I found on a google search for a 2005 super duty...
Before deactivating/activating the Belt-Minder®, apply the parking brake.
Place the gearshift in the PARK (P) position (automatic transmission) or the NEUTRAL (N) position (manual transmission).
Place the ignition switch in the OFF position.
Close all the vehicle doors.
Unbuckle the driver safety belt.
Place the headlamp switch in the OFF position.
Turn the ignition switch to the RUN position (do not start the engine).
Wait until the safety belt warning indicator turns off (approximately 1 minute).
NOTE: Step 3 must be completed within 50 seconds after the safety belt warning indicator turns off.
At a moderate rate, buckle then unbuckle the driver safety belt 9 times, ending with the safety belt in the unbuckled state.
After this step, the safety belt warning indicator illuminates for 3 seconds.
NOTE: The buckle/unbuckle sequence may not be counted if completed too fast. Wait approximately 2 seconds after each sequence.
Within 10 seconds after the safety belt warning indicator turns off, buckle then unbuckle the safety belt.
This disables the Belt-Minder® feature, if it is currently enabled. As confirmation, the safety belt warning indicator flashes 4 times per second for 3 seconds.
This enables the Belt-Minder® feature, if it is currently disabled. As confirmation, the safety belt warning indicator flashes 4 times per second for 3 seconds followed by another flash sequence of 4 times per second for 3 seconds.
After confirmation, the deactivation/activation procedure is complete.
Or you can unplug the harness... that should work too