This question has been posted here before. If I remember correctly, there is NO way to disable that. I know what you mean, though. It IS a pain. My X5 ('03) does not have that, thankfully; my wife's X3, however does. I'm like you, I wear my seatbelt. BUT, It aggravates me when I am just driving three blocks to pick up the kid from his friend's house, and that chime starts going off...
Juan