Requirement
You want to exclude one DB during selected Mailbox store: Automatic mailbox Distribution in the Active Directory Account Template.
Identity Manager 14.5, 15.X
Currently there is no OOTB solution to do it in IM. You can either select single Mailbox database to be used or all of them.
IT seems Exchange command:
Set-MailboxDatabase with -IsExcludedFromProvisioning paramether
should help here from Exchange side.
You can create enhancement request to address lack of this functionality from IM side.