How do you remove calcium deposits?

Calcium can be broken down with an acidic solution of some kind. There are cleaning agents sold which are made specifically for this.

You can remove calcium deposits using baking soda and white vinegar, depending on what kind of surface you are cleaning, you can scrub the area with either steel wool or a nylon scrubber. This is a lot less expensive than store bought products. You can also use salt and vinegar.