In order to turn off new definition scans do the following:
1. Open the Symantec Endpoint Protection Manager.
2. Select Policies from the far left.
3. Highlight Antivirus and Antispyware under the View Policies section in the middle column.
4. Highlight the active policy from the right side of the manager.
5. Click Edit the Policy under the Tasks menu is the middle column.
6. A new window will open.
7. Select Administrator-defined Scans from the far left menu in the new window.
8. Select the Advanced tab.
9. From under Startup and Triggered Scans uncheck Run an active scan when new definitions arrive.