To find clients that are Offline:
1.In the console --Monitors -- Logs-- Computer status--Advance Settings--Set specific dates which you want from and to.--
2. Make computer status Online status to Offline
After report generated you can remove manually offline clients.
If you have Domain Controller in your network
1.Add the Domain to your SEPM server
2.You cannot delete items that have been imported from AD.
3.You can only sync by right clicking on the AD group where the machine appears and select "sync now".
The SEPM does not have the ability to manipulate any AD items. It synchs with AD and must be resynched to reflect changes.