Our support folks have created a tool that enables you to remove permanently offline computers from your FCS database.
Client computers go offline for many reasons, including decommission, re-tasking, and re-imaging. After a day, these computers are shown in the "Not Reporting" section of the Client Security console. After 30 days, these computers are removed from the statistics in the Client Security console, but will still be visible in the Connectivity report as well as in the Operations Manager Administrators console.
The Forefront Client Security Offline Asset Removal Tool (OART) is a supported command line utility used to remove both MOM computer objects and the discovery rules for those computers. You must run the tool on your collection server.
Hope this is helpful!