Your toilet should be cleaned regularly so that limescale deposits are kept to a minimum. However, if you have extremely hard water, even regularly cleaned toilets will likely have a problem with limescale. There are safe and non-chemical ways to remove limescale. You will want to use a cleaning agent that will not harm the porcelain.
Clean the toilet bowl with a toilet bowl cleaning product to give the bowl a general cleaning. Flush the toilet upon completion.
Measure out 3 cups of white distilled vinegar and pour it into the toilet bowl. Measure out 1/2 cup of baking soda and pour it into the toilet bowl.
Leave the vinegar and baking soda in the toilet bowl overnight. This will give the vinegar and baking soda time to work on the limescale.
Make a sign that says, "Do not flush," and tape it to the lid. This will serve as a reminder not to use the toilet.
Scrub the toilet with your toilet bowl scrub brush. Flush several times to clean off any loose limescale.