you cant. the power button overrides all passwords.
yes you can stop users from shutting the system down/sleep via windows by setting up a user account. but really what you want to do cant be done because you cant disable the power off button.
There is actually an option to do that. It's a rarely used option but it is indeed possible
i just dont want people to shut down my computer when im not sitting in front of it
Take a look HERE
in regards to how to change what the power button reacts when it's pressed. There is also a secondary thread in that link that will guide you through removing the hibernate and stand-by buttons entirely.IMPORTANT NOTE:
Local group policy is only available in Windows 7 versions Professional and above. This option does not exist for Starter or Home Premium.