If possible, reduce atmospheric moisture and overly shaded conditions, though you may find certain species of lichens on rocks in full sun. If lichens seem like an unsightly problem for you, they can be controlled somewhat with a copper fungicide. Another is the algaecide copper sulfate. Follow the label directions for general foliar disease control. You may have to repeat applications if the lichens continue to grow and spread.
Many gardeners consider lichens attractive and actually encourage their growth.
It is also a good idea to check the general health of plants that are colonized by lichens. Vigorously growing plants that are well pruned and in light, airy situations are rarely seriously troubled by lichens.