To prevent users from disabling Symantec Endpoint Protection (SEP) on their client:

Step 1: Remove the right to disable Network Threat Protection:

1. Open the Symantec Endpoint Protection Manager.
2. Click Clients.
3. Select the group that contains the clients you want to be affected.
4. Click Policies.
5. Expand Location-specific Settings.
6. Click Tasks to the right of "Client User Interface Control Settings", then click Edit Settings.
7. Select Server control or Mixed control if it is not already set to one of these.
8. Click Customize.
   • If Server control is enabled this will open the Client User Interface Settings dialog.
   • If Mixed control is enabled this will open the Client User Interface Mixed Control Settings dialog.
     
      
9. Uncheck Allow users to enable and disable Network Threat Protection.
10. Click OK> OK.

Step 2: Remove the right to disable Threat detection:

1. Open the Symantec Endpoint Protection Manager.
2. Click Clients.
3. Select the group that contains the clients you want to be affected.
4. Click Policies.
5. Expand Location-specific Policies
6. Click Antivirus and Antispyware policy.
7. Click File System Auto-Protect, then lock this feature by clicking the lock symbol next to Enable File System Auto-Protect.
8. Click Internet Email Auto-Protect, then lock this feature by clicking the lock symbol next to Enable Internet Email Auto-Protect.
9. Click Microsoft Outlook Auto-Protect, then lock this feature by clicking the lock symbol next to Enable Microsoft Outlook Auto-Protect.
10. Click Lotus Notes Auto-Protect, then lock this feature by clicking the lock symbol next to Enable Lotus Notes Auto-Protect.
11. Click TruScan Proactive Threat Scans, then lock this feature by clicking the lock symbol next to Scan for trojans and worms and Scan for keyloggers.
12. Click OK.

For Symantec Endpoint Protection 12.1, additional policies must be locked. 

1. In the Virus & Spyware Protection policy, click Sonar, then lock this feature by clicking the lock symbol next to Enable Sonar.  
2. In the Instrusion Prevention policy, click Settings, then lock both lock symbols next to Enable Network Intrusion Prevention and Enable Browser Intrusion Prevention.  

Step 3: Force clients to update policy:
This step is not necessary as clients will receive the policy during their normal heartbeat, however, you may be able to speed up the process by performing the following:

From the Symantec Endpoint Protection Manager:

1. Open the Symantec Endpoint Protection Manager.
2. Click Clients.
3. Select the group that contains the clients you want to be affected.
4. Right-click on that group.
5. Arrow over Run Command on Group.
6. Click Update Content.
7. Click Yes> OK
   The client will receive a prompt to heartbeat and update its policy. Once the policy has been updated the option to Disable Symantec Endpoint Protection will be grayed-out when users right-click the Symantec Endpoint Protection system tray icon.

On the client:

1. Right-click the Symantec Endpoint Protection system tray icon.
2. Click Update Policy. The client will request the new policy from the manager

Once the policy has been updated the user will not be able to disable the Antivirus/Antispyware or the Network Threat Protection features. 

Hope this helps.

Link for above information:

http://www.symantec.com/business/support/index?page=content&id=TECH102822