There are several ways to go about shutting off individual spray nozzles and a lot depends on what type of nozzle body you are going to use. One narrow boom I have has small ball valves in the hose between the nozzles. If just using the old style nozzle bodies with screw on caps you can use blank that replaces the tips.  On the bodies that use the quick caps you can get a shut off cap.  On the nozzle bodies that use diaphragm check valves there is a replacement cap for the diaphragm that lets you turn that nozzle on and off.