There is a procedure to disable BeltMinder (That is what Ford calls it)
I disabled it in my Econoline, because it seems they made it more annoying in commercial vehicles... As soon as one takes their seatbelt off after driving for a few, instantly bothers you, even in park. The Taurus I never bothered to disable it since it shuts up when the vehicle is in park typically.
The procedure is typically buckling and unbuckling the belt like 8 times, and something... The info is in your owners manual or available online at www.fleet.ford.com
i'm glad to have found this thread....keep in mind, I'm a firm beleiver in wearing a seatbelt....but like you said, you take it off after you put in park and the engine still running...and off goes the stupid dinger! I'm also interested in learning to turn just the dinger off.