From the Map Layers editor, under the Layers menu click Remove Map Layer.
The Remove Map Layers from Watersheddialog will open. Select the map layer you wish to remove from the watershed and click Remove. A warning window will display asking you if you really want to remove the selected map layer, click Yes.
The Remove Map Layers from Watersheddialog will close, and the selected map layer will no longer display in the Map Layers editor.