To delete an existing custom Search Scope for your application:
Navigate to the App Designer (Settings > Applications > App Designer), then click the Search Scopes option (under User Interface). The Add and edit search scopes screen appears:
Select an existing search scope and click Delete. A message appears confirming that you’d like to delete the selected scope. Click OK to confirm and continue.
The screen refreshes and the deleted scope is no longer listed with the search scopes.