My car does not have a belt warning bleep but it does have a lights on alarm. I inserted a timer module so it only bleeps for a few seconds.
Obviously you need to be able to get to the bleeper module to do that (it might be integrated into your dash electronics in which case it would be difficult)
Alternatively if you can locate the piezoelectric bleeper then you could also reduce it's volume with an inline resistor or just some tape over the bleeper.
