And, by the way, this seems like a brainless and punititive way to solve the usage problem. If you really wanted to save water, as opposed to just making people miserable, get rid of the tub spout and just have a showerhead that can be used to fill the tub. In many, if not most tub/shower combinations, much of the water flows uselessly out of the tub spout and is wasted.
Unscrew the shower head, look inside for the little plastic flow regulator thing, rip out with pair of needle nose pliers.
Problem solved!