hi Shaheen,
you can deactivate org unit by ending the validity please fallow below steps to do so.
go to org managemnt work center.
select edit org structure under common tasks
set effective from date as required
select the org unit ABC.
select edit org unit and choose end validity
select save button
select check button choose check all option.
note:-if you try to deactivate the org unit earlier than today then this cannot be allowed as there might be new docuemts created.
in this case please go to timelines tab in org unitenter todays date or future date into the valid to field for row org unit timeline and hit enter
Regards,
Suresh