One way to disable the alarm is to pull the seat belt buckle out of the receiver and then place it back into the receiver very quickly but dont do it until you hear a click to lock it in place. This should temporarily disable the sensor and keep the seatbelt light from flashing/buzzing.
Another way to disable the seat belt alarm is by turning the ignition key to the "ON" position and then pressing and holding the passenger side air bag switch to the "ON" position for at least ten seconds. This should turn the passenger side seat belt alarm off.
If neither of these methods works, you may need to take your vehicle to a mechanic to have the passenger seat belt alarm disabled.
It is important to note that disabling the passenger seat belt alarm is a potentially dangerous modification. If you are considering doing this, please be aware of the risks involved.