You can easily twist the Piezo chime off the circuit board with a pair of pliers. This will end the seat belt chime. But also ends the reminder that you left the keys in the ignition or left the headlights on.
If you disconnect the under seat wires, you may affect the SRS auto tensioning system so I would be careful on that one.
The seat belt minder chime can be turned off.
Read Seat Belt Minder in your owners manual
My 2011 Ranger Owners manual procedure
says to buckle and unbuckle 9 times.
The lights did not flash appropriately and
the procedure did not work.
I read another set of instructions that said 3 times
so I tried it and it worked the first time.
Hate those damn chimes too.