Cannot Delete Exchange 2010 Database because of arbitration mailboxes

If you run the command: Get-Mailbox -Arbitration -Database DB
you see arbitration mailboxes listed.

Move them - Get-Mailbox -Arbitration -Database dbsource | New-MoveRequest -TargetDatabase dbtarget

You can also disable them unless it is the first DB in the org by doing:
Get-Mailbox -Arbitration -Database db | Disable-Mailbox -Arbitration