Leaf spot usually occurs from sitting water on the leaves and can also be a sign of overwatering.
As with all other fungal and bacterial diseases, better air circulation, well-drained soil, dry leaves, and less water help control leaf spot. Avoid letting water sit on the leaves from overhead watering or heavy misting. You can use neem oil or a liquid copper fungicide to treat leaf spot and always follow the directions on the bottle. The spots will not turn green again, but treatment will help them from spreading.
Prune off any heavily affected leaves off with clean scissors or shears and disinfect the blade between each cut. Be sure to keep infected plants away from your other houseplants. Never mist a plant if leaf spot is suspected as it can help it spread.
Water only when the top 50-75% of soil is dry. Water thoroughly until it flowers out of the drainage hole and discard the excess water.