Under Setup > System Administration > Monitoring you can customize the Unusual Detection to some extent, but it's not possible to disable it for Unusually Low only. It's also not possible to re-configure this for a specific sensor type only.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.