There actually is a rather low-tech measure that works very well: there's a plastic container bag that comes with every ez-pass transponder, and is coated with RF-blocking material. It looks a lot like very sturdy, semi-trasparent foil. The EZ-pass instruction booklets tell users that if they are concerned for any reason that their transponder might be read in an instance where they don't want it to be, they can remove the transponder from its windshield mount and place it in the bag, ensuring that the transponder will not be read.
Not as convenient as an off switch, but it does solve the problem of people not being aware of the unit being "off" when they in fact want it "on." If it's in the bag, it's off. If it's on the windshield, then it's on.
E-mail fudged to thwart spammers. Transpose the c's and a's in my e-mail address to reply.