Each object is given a ElementID (GUID) when it is published to the PPSMonitoring database repository. These are then used to create the published folder in the Preview directory and I would have to assume that this is how the items are ordered. If you have dashboards that you no longer want to see displayed on the Preview site then delete the folders in the Preview directory.
If you installed on the c-drive then you would want to look here - C:\Program Files\Microsoft Office PerformancePoint Server\3.0\Monitoring\PPSMonitoring_1\Preview
Here you can query for the dashboards in the repository:
SELECT [ElementId]
,[Version]
,[ObjectType]
,[Name]
,[Description]
,[Owner]
,[Created]
,[SerializedXml]
FROM [PPSMonitoring].[dbo].[FCObjects] where ObjectType = 6
Nick Barclay has a tool that you could use to get some inforamtion out of the repository on CodePlex that might be of interest to you -
http://maudf.codeplex.com/.
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