At the moment there is no option to mark documents as obsolete.
Once a document has been created in the Documents menu, Self Service folder, it is possible to either delete it or uncheck the Requires Acknowledgement box.
This can be done by following the steps below:
Click Setup.
Go to Content and click Documents.
Click the Self Service folder and then on the document you wish to edit.
Click Edit in the top right corner.
Uncheck the Requires Acknowledgement box so that the document won't prompt users to acknowledge any longer.
Click Delete to remove the document from the system.