I'm assuming you have a separate OU for users, one for Servers, and one for Computers. If you do, you can set a GPO on the user OU to disable the Turn off computer button. Then you can set a GPO on the Computer OU to enable the Turn off computer option. Then can enable Loopback Policy on the Computer OU GPO and select Replace mode. This will override the user GPO with the computer GPO. Here is more detail:
http://support.microsoft.com/kb/231287