Disabling the warning when fitted would be an instant MOT failure. As Colin says Why? do you want to disable the alarm.A seat belt alarm is a nuisance but it does help when forgetting to put the seat belt on. Sorry to bring this up but anyone that has had anything to do with the aftermath of a accident where the seat belt was not worn would put up with the noisy nuisance.
If its because you carry goods on the front seat, why not just connect the seat belt which will obviate the noisy little beggar. If this is the case and the object is heavy, why not put the seat belt around it, in the sad case of a crash the last thing you want flying around in the car is a heavy lump.