Hello and welcome to Just Answer, my name is Matt.
The most likely cause for this problem is a faulty seat belt buckle (the part with the release button on it). There is a switch inside that closes when the seat belt is bucked.
Another way to take care of it...unbolt the seat and inspect the wiring under the seat. Look for a yellow connector with 4 wires. On the 2 wires that go to the buckle itself, if you strip a bit of the insulation and touch them together, this eliminates the problem. Be sure not to do this to the wrong wires...the other 2 in the same connector go the pretensioner....shorting these together will cause the airbag light to come on. Be sure to disconnect the battery before doing this type of work.