Just to elaborate on post by yeahthatsme. To temporarily deactivate the seat belt reminder, turn the ignition switch to the ON position, fasten the driver's seat belt for 2 seconds or longer, and then unfasten it within 20 seconds. Belt minder will be deactivated until the ignition switch is turned off and then back to ON.
Long term or permanent deactivation can only be performed with the dealer diagnostic IDS computer. Is a very simple procedure taking about 3 minutes but they will probably want to charge an hour or at least that seems to be the going rate. Check out this earlier thread covering the same topic scooby420 for even more information about this.