There is currently no unshare function but if you wish to no longer share an album, you can make a copy of it and delete the one you had shared.
- First open the album and click "SELECT ALL".
- Then click "Options" on the left, and select, "ADD PHOTOS TO ALBUM" (see below).
- From there, type in a new album name to create a copy.
- Finally, re-open the originally shared album, select all the photos again, and in the Options menu, click "DELETE PHOTOS".