Actually immutability is a property on the device and not on the Data Protector side. Data Protector has slots and media linked to the immutable objects on the store only. This means it is possible to delete media and slots which are linked to immutable store objects when the media have no protection. One way to avoid that is to make sure that the protection in Data Protector is equal to or higher (longer) than the immutable period on the device. 

Another more "waterproof" way is to make use of the following global parameter.
(ProgramData\OmniBack\Config\Server\Options\global or /etc/opt/omni/server/options/global)

# DeleteUnprotectedMediaForce=0 or 1

# default: 1
# By default expired B2D media are always exported, with all corresponding
# catalog purged from IDB, even if deletion of corresponding data fails -
# ensuring cell's catalog does not accumulate in case data is no longer
# accessible on store (e.g. due to manual deletion).
# If disabled (0), expired B2D media will not be exported from IDB if
# corresponding data deletion on store fails.
# The default is enabled (1) to ensure expired catalog doesn't get stuck on
# the cell server if data cannot be deleted for whatever reason.

The comments above do explain the downside of disabling this parameter (DeleteUnprotectedMediaForce=0). However, when immutability on a B2D device is used, it may be concidered to disable it as this ensures that the media and slot won't get deleted as long as the object is not removable on the device side, which is the case during the immutable period. Indeed, as long as the object is within the immutable period, and error will be seen when trying to delete the related media. Depending on the setting of the parameter, the media will or won't get deleted from the IDB.

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