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try changing the permissions for the group "Anonymous Users". Also, if you're not supporting sign ins, you can remove the "Login" block (don't disable access to the user.php page, though. you still need to log in as admin). Finally, i may be wrong... but if i remember right, i think disabling new registrations is an option under general preferences.... check the options on that page and see if it's there.
hope that all helps. good luck with your site!
-zimmi88 =)
"Need coffee, keep away..."