Tell her to leave the handbag in the car, if possible.
If carry any empty plastic bag into your pocket. If she pulls this on you again, walk out of the store, put the handbag in the plastic bag, n walk in like nothing happened. Or tell the attendant in the ladies changing rooms to inform her that you've gone for a smoke, n take your own sweet time smokin'. She can't go anywhere cos you've got her handbag. : )