1. Is there any way in Symantec Endpoint Protection Manager 12.1.6 to stop administrator defined scan or schedule scan ?
2. Is it possible to configure SEP clients, so that they can see/stop a schedule scan or administratirve scan ?
Yes, to both.
From your AV policy go to "Administrator-Defined Scans" tab and select the Advanced tab
Under Scan Progress Options you have multiple options for what you want to do as well as set the Pause options. You should be able to accomplish what you want here.