There is a chemical you can buy and use, Lime-Away. This works on shower heads as well. Be sure the room is well ventilated, it's not good to breath. Another product, I forget the name but the ads show brushes with eyes on them scrubbing the bathroom and use the term "scouring bubbles" claims to do the same thing.
Natural stuff is lemon juice and salt, or baking powder, a little water and a toothbrush, but be sure you dissolve the salt or it will scratch. Takes a lot of work to get it done. Ammonia may also do the trick, it's used in store products to clean burned on food off of pots and pans. Also need to ventilate the room, it's pretty strong.