1.) The Theft Deterrent Module, way up under the left-hand side of the dash, has a 20amp in-line fuse right next to it. Just pull the fuse to disable the system...no fuss no muss.
2.) Are you using a key fob? If so, you might need to reprogram the fobs. Instructions are in the Reatta Owners Journal.
Reatta Owners Journal - The Buick Reatta Community