Use a long-handled, stiff brush dipped in water to scrub away as much of the lichen as possible.
Spray a fungicidal solution onto the deck surface. Follow the directions on the solution to create the cleaning mixture.
Allow the fungicide to sit on the deck for up to 15 minutes or according to the time stated on the directions. If possible, allow the solution to sit on the deck for up to three days, if the weather is dry and sunny.
Add water and scrub the deck with the long brush to loosen and remove the lichen.
Rinse the area thoroughly with water from a power sprayer or garden hose.
Keep the deck clean and routinely brush and rinse the surface to keep lichen from spreading.