Having an '04 Suburban, I've become all too familiar with the incessant dinging as well, for over 3 years now. And also came to know that there really is no way to turn it off, trick or otherwise, it's just programmed into the system and wiring, and just "like that" from now on--unless you feel like researching and cutting wires, etc.
For a while, I didn't see anyone really come up with a solution, UNTIL I just read on a GM truck forum that someone just unplugged a small harness under each front seat that goes to the seatbelt--therefore killing the chimes. And so far, no ill effects.
This was on a 2007 GMC Sierra, and there's a lot of electrical similarities in how they're set up compared to an Acadia/Outlook/Enclave, but no for sure answer on making a similar move with one of them--but it could be similar.
In the meantime, I'd check out the much more active GMC Acadia forum and also go over to the Outlook forum at SaturnFans.com and ask about something like this^^, as far as just unplugging something. You'll likely just get a lot of "no, can't be done...annoys me too" responses, but you never know, since there's always a few people that experiment with everything.