The only thing I can think of that might help would be a good scrubbing with alcohol or vinegar as both these do remove hard water stains. It may take several times and some elbow grease to get it off. After removing the hard water spotting, try protecting the area with paste wax or rain-x which can be found in the automotive department in department stores. You will have to reapply every month or so as it does wear off.

I've done the same for my bottled water grill as I used only spring water which contains lots of minerals. For lack of rain-x or paste wax, I used future floor wax instead. This method also worked except that eventually it started to peeling off rather than wearing off. The peeling didn't bother me so much as the yellowing that future gets after a while. I'd then re scrub and reapply... yeah yeah, I should have gone out and gotten the proper stuff but I was trying to use up my useless "Future" since I had no wax floors by that point LOL!