If you have intergrated your AD ; you will not beable to move or delete the client from the list. Can you please check if you actually have one such user/computer in AD structure... If you still want to delete the client ot user, you can probably run a query...
delete from SEM_CLIENT where compter_name='name_of_client' AND deleted='1'
Note: Please run a select query before you actually delete, just to make sure you delete the stale entry.