This will have to be a mechanical fix, and cannot be done through software as seat belts are mandated by law.
I have not done this in a while but the absolute easiest way to defeat the reminder is to go to the auto wrecker and get an identical male end of the belt and cut off the fabric leaving you with just the buckle and insert the buckle into the female receiver of the drivers belt.
You could also drag over the passenger belt and twist the belt head around to defeat the drivers belt sensor. I would not recommend this as it will distort the passenger belt.
Other then that, the female part of the buckle will have a sensor that is normally open or closed. when the seat sensor registers your weight. By either jumping or disconnecting one of the two wires will disable the sensor into thinking that the belt is always connected.
Personally being that the belt is one of these federally mandated items (similar to labels on pillows and mattresses) I would not alter the wiring and go with the extra buckle connector to fool the belt into thinking it is attached.
The difference between the two is that anyone else getting into your car and wanting to use the belt will see the extra piece in place and remove it and then attach the belt as normal, by altering the wiring if the reminder is not on and they get in an accident without wearing the belt you are liable as they can claim the reminder was altered and because of this alteration they forgot to put the belt on.