By default, the FortiClient will start up automatically when starting macOS. This may not always be desirable but fortunately, auto-start can be disabled quite easily.
The FortiClient launch agent is located at /Library/LaunchAgents/com.fortinet.forticlient.fct_launcher.plist. Use any text editor to open and edit this file, but you probably will need to do this using Terminal and using sudo to launch a text-based text editor (e.g. vi or nano).
With the file open, look for these lines:
Change true to false and save the file. Note that updates to the FortiClient may overwrite the change.
When you're using your Synology exclusively as a iSCSI target, and have assigned the complete disk space to your LUN's, you will get notifications about the volume being (almost) completely full. Obviously, in this case, you may not want to receive these notifications. Luckily, you can turn these off:
Note that this will only disable the notifications (e-mail, SMS, etc.). The widget and WebUI will still show the alerts.