Hi Bhagee,
Please go through one of my feedback on the below thread, and take clues form it.
http://scn.sap.com/thread/3724704
Hope this works for you to set UD with background stock positing.
NOTE: Please note that if someone has access to UD (QA11), then I think he can still go to 'Inspection Lot Stock" tab and still post manually. So to refrain users completely navigating into this tab, you can try working with authorization team to block this tab, or implement a transaction variant for QA11 where this tab is missing.. either of these two proposals are not tried before, so you can investigate it further and update here for the sake of larger audience.
Thanks,
Arijit