Sometimes the extension can be toggled off directly from the SEP client software on the taskbar/system tray.
If you cannot disable the extension directly in Chrome, the next best option is to tell the SEP desktop client to stop injecting the plugin. This is the "proper" way to do it without hacking Chrome flags. disable symantec endpoint protection chrome extension
You may need to enter an administrator password if your IT team has set one. If you don't know it, you cannot proceed. Sometimes the extension can be toggled off directly