down spout to shower problem

To turn on the shower I have the type where there is a small pull knob on the top surface of the downspout to divert the water to the shower head. The water is "leaking" through the downspout reducing the pressure to the shower head. Is there something that is replaceable to correct this problem? Thanks.

Replace the spout!

Spouts are generally two types: screw on or push-on. If there's an opening on the bottom and a set screw, it's probably a push on. Loosen the screw and pull it off. Take it to the store and pick up a new one.
Screw on are tougher since the fitting has to be in the right place so it tightens up the proper distance from the wall.