With existing crabapple trees you can do nothing and let the tree defoliate each summer. Apple scab is generally not life threatening for the plant, but certainly lessens its ornamental appeal unless you are really into bare branches. As with other diseases, try to keep plants healthy by watering during drought and fertilizing periodically. Following good sanitation practices may help. Remove and destroy infected leaves, flowers, and fruit as soon as possible.
Second option is a fungicide program.
There are several fungicides labeled for apple scab control such as captan and chlorothalanil, often sold as Daconil. Be sure to read and follow all label directions and precautions. Begin fungicide spraying as leaves develop and continue according to label intervals until frequent wetting by rain has lessened, usually by July 1. If some spray intervals are missed, apple scab would still be lessened but complete control may be lost. Remember fungicide sprays are predominantly protectants against infection so new leaves have to be sprayed before infection occurs.