How to keep calcium out of coffee maker?

Vinegar overnight. Google has the details.

Fill the coffee maker 1/2 with white vinegar 1/2 filtered or distilled water. Brew halfway. Then let it rest for 1/2 hour. Brew the rest. Flush out the coffee maker by brewing distilled or filtered water 2 to 3 times. Works well. Brew coffee with filtered water, never tap water.

Baking soda

Normal filtering will not take out the hard water deposits. Softened water has less deposits - but you need distilled or highly demineralized water to not get hard water deposits.

For cooking and drinking water my house uses the hard water that is run through a filter so it tastes better (I actually have a very expensive water purifier that also kills bacteria which is way above the capabilities of most water filters: Note there are 3 or 4 different water filter certifications: Britta and many others are just certified to improve tastes - not to remove many harmful things).

My highly filtered and purified water still produces hard water scale in the teapot and in pots we use for cooking; because normal filtration does not remove dissolved minerals in water.

I suggest that you just get used to periodically dissolving out the had water deposits out of normal cookware and coffee/tea pots. For medical device such as a CPAP humidifier or for Irons (ironing clothing) distilled water (sold in gallon jugs at most grocery stores).

Pour the water into the coffee maker using a water-filtering jug. That's what my housemate does. Hers is Brita brand but there are other brands and knock-offs. The filter part has a cartridge that can be replaced when it wears out.

I prefer a French press. Much easier to keep clean.

Either run vinegar through it every once in a while or get a water softener.

Pour the water through a preva filter.