To remove an orphaned or decommissioned server from a Windows SharePoint services farm, do as follows:
Open ‘Central Administration’ for your Windows SharePoint Farm. Click “Operations” then under ‘Topology and Services’ click ‘”Servers in Farm” then click “Remove Server” next to the name of the server you would like to decomission from the farm.
Note that the recommend way to remove a SharePoint server is to uninstall all SharePoint Services from the server using Add/Remove programs. You should only decommission a server using the Central Administration option when the server is no longer available.