suppose that i remove the domain admins group from SQL Server 2005 security tab.?
Still don't understand what you mean... are you referring to BUIILTIN\Administrators? Do you mean removing them as a SQL login? or removing them from the local Administrators group of the Windows server concerned?
This means that no administartors have access to SQL Server databases and other SQL Server 2005 resources. But can they still shutdown SQL Server or restart it's services?
Windows Administrator's will always be able to Start/shutdown or restart/shutdown a given Windows server.