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:

1. Log on to your DSM.
2. Navigate to Control Panel → Notification → Advanced tab. In the list, expand Internal storage.
3. Uncheck the notifications for Volume in low capacity and Volume in extremely low capacity. Click Apply to save the changes.

Note that this will only disable the notifications (e-mail, SMS, etc.). The widget and WebUI will still show the alerts.