Unfortunately I'm not aware of any way to completely hide sheets that are not being used as part of a dashboard on Tableau Desktop, however it is possible to collate these into a single tab. You can do this by creating a new dashboard and dragging in each of your unused sheets, then hiding all the sheets for that new dashboard. Call that dashboard "Unused Sheets" or something similar.
Apologies that this does not completely hide the sheets however it does mitigate the issue.
When publishing to Tableau Server however you can avoid showing any undesired sheets using the tick boxes, as in the screenshots below (image for Tableau 10.2).
Let me know if you have any questions on this.