A Data Source in Discover is the high-level data repository that can be accessed and processed by an Discover agent. A Data Source is made up of Data Targets which contain the data that is ultimately managed by Discover. So for example, Exchange Mailbox is a Data Source and each of the mailboxes managed by Discover serve as Data Targets.

You can delete a Data Targets by selecting a Data Source, hovering the Action button, and selecting Delete Targets.