Just to be clear, you're talking about a user creating a new workbook from scratch using the web authoring feature on Tableau server only aren't you? And once that user has saved their workbook, you only want them to have the rights to access it... hmmmm...
I would have thought that provided they have saved the workbook in a project where the permissions are 'managed by the owner' rather than 'locked to project' (see here Manage Projects and Project Permissions ), then the owner should have the right to edit the permissions of the workbook online too after they have saved the workbook (View or Edit Permission Rules and User Permissions). However, note that regardless what permissions the user sets, site & server admins will still be able to access that workbook.
PS - I said 'should' as I'm server admin so can't test this theory as I have uber rights :-)