I have an iMac running Lion. Non-admin users are asked for an admin username and password when they try to shutdown or restart the Mac, but not when they sleep.
How do you deny permission to put the iMac to sleep for non-admin users?
You could set the energy saver pref to "never sleep" and lock that pane so they cannot change it. That would keep it from sleeping on its own, but I am not aware of a way to stop then from manually sleeping it.