Now obviusly the first thing that can be done is softening the water, but that is not always a viable option for homeowners. If this route is taken you will have to remoove the existing scale, the same method for regular maintenance.
Depending on the level of scale, this may take multiple applications to remove, but it is still a great idea to impliment into your cleaning ritual every month. We like to beigin with 0000 Steel wool, now this isnt the same as what you would use to clean pots, this product is very fine, and made for glass surfaces so it will not scratch the glass, I promise... Its only good for one time use, the fibers are very fine and will rust within a couple of days.
Once you have your fine steel wool, dip it in a mixture of water and soap, easy on the soap, were just using it as a lubricant to help it glide smoother so the soap isnt releavant either, in a circular motion start from the bottom and work your way up applying light pressure and let the product do the work. Follow up by cleaning the glass as normal. For lighter stains the results will be immediate, for harder stain build up it will take multiple applications.