I got this from the help files, never having personally initiated sharing of a workbook (I've used them, however):
Remove a workbook from shared use
If you decide that you no longer want others to make changes to a shared workbook, you can open and work in the workbook as its only user. When you remove a workbook from shared use, you disconnect all other users from the shared workbook, turn off the change history, and erase the stored change history so that you can no longer view the history or merge this copy with other copies of the shared workbook.
Caution To ensure that others do not lose work in progress, make sure that all other users have been notified so that they can save and close the shared workbook before you remove it from shared use.
On the Tools menu, click Share Workbook, and then click the Editing tab.
Make sure that you are the only person listed in the Who has this workbook open now box. If other users are listed, they will lose any unsaved work.
Clear the Allow changes by more than one user at the same time check box, and click OK.
When prompted about the effects on other users, click Yes.
Hope that answers your question -- if not, post back, and somebody who's confident about this feature should get back to you soon. Looks like it should work, though.