We recommend that you take the plants into a utility sink and wash the plants off with water. Be sure to wash off the leaf undersides. The leaves are large enough that you should be able to monitor for the spider mites. If not effective, then spray with insecticidal soap, the least toxic option. 

Also, focus on proper care.  These plants can be a bit fussy about growing conditions. They like humidity and you will have to keep up with it. Mites like dry conditions. 

The plants like medium light (for example, a tree shaded window).  Brighter light often spoils leaf coloring. A temperature between 60-70 degrees is ideal. In warmer rooms high humidity is essential and the foliage should be misted daily. Or stand the containers in a shallow tray in which a layer of pebbles has been placed. Pour water over the pebbles  but do not let the water level rise above the pebbles. During the active growth period water to keep the potting mixture thoroughly moist. In the winter months - water enough to make the mixture moist but allow the top half inch to dry out between waterings. Use room temperature water. They should be given standard liquid fertilizer every two weeks during the active growth period only. Healthy plants need to be moved into pots one size larger every year in late spring or early summer. Here is a link to more information https://www.ourhouseplants.com/plants/calathea

As a last resort, if the above is not effective, you can look for a houseplant insecticide that is labeled for spider mites and houseplants. Read and follow label directions.

Marian