How do I disable the seat-belt sensor for the passenger seat in my 2012 Holden Colorado?

A better idea would be to fix the sensor that’s creating the false alarms. That will probably be simpler and quicker than finding the right wire to cut or fuse to pull out, as well as maintaining the car’s legality. A car that doesn’t have all its safety equipment functioning properly is, technically, unroadworthy and if that fault can be blamed for any injury in a crash, then you could be in real trouble in a legal and insurance sense.

Don’t forget, too, that a car such as yours will usually have a sensor to determine whether the seat-belt is secured and another to determine if the passenger’s seat is occupied. Either of those could conceivably be causing your false alarm.