I would like to disable the chime as well, but I want to qualify:

I am 100% for seat belt use and have used them way before usage ever became law.

BUT, as we approach our house my wife likes to take hers off a few feet (sometimes more) before we pull into the driveway. The dinging is crazy annoying and unnecessary at this point.

Clublexus.com has a post on how to disable the buzzer on the lexus. I tried it out and it does not work. Guess I have to have the dealer do it.