Right, you need to have the H9 connected to the app to deregister it. We do things this way, because the registration of the H9 is one of the most important safety mechanisms we have to prevent just any from connecting to your H9 over Bluetooth from H9 Control and changing its settings while you're using it. But in any case please send an email to support@eventide.com with the serial number of the H9 you would like deregistered.
