Hi,
Thank you for posting in Symantec community.
You cannot stop the SepMasterService, you can only restart the SEP client service.
To restart go to Start--> Run --> smc -stop & smc -start (It will restart the SEP client service.)
As per comment last policy is not working, can you repair the SEPM?
Chec this article as well:
How to prevent SEP features from being disabled in the client GUI in SEP 12.1
http://www.symantec.com/docs/TECH168990