You want to prevent SEP service from starting automatically at boot for test purpose.
You can use smc -stop if this is not enough and need to totally disable SEP service from running:
Disable Tamper protection as per :
Protecting Symantec Endpoint Protection processes and resources with Tamper Protection
Navigate to [HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\services\SepMasterService]
Set the Start to 4 as Disabled so next boot the Service won't start automatically.
Each value stands for the following: