There are a few precautions you can take to prevent people from simply "unplugging" or "switching off the socket" - cover the switch and hide/lock the cables away. You can get special PC cages that allow you to secure the computer and it can often be used to make sure people can't unplug the power from the rear of the computer. The other end of the power cable (the plug and wall socket end) is much more difficult. You could build a cabinet that you lock the PC away but also have the cabinet cover enough power sockets for you to power the PC and the monitor. Cut a hole just for the power button/any front ports you need access to (CD-ROM, USB etc.).
As @Dave pointed out his in his comment, you will need people to have access to the power button because if you want them to be able to shutdown the computer, then you'll also want them to be able to turn the computer on again the next day/whenever it's off and needs to be on - unless you make this a task you complete yourself, in which case you can lock the PC away and only you or authorised people can turn it back on again. even pressing and holding the power button will turn the PC off as if you'd pulled the plug - Windows will not have had chance to shutdown gracefully.
Without physically locking the computer (and the wall sockets) from general public access you'll never be able to solve your problem. Other solutions involve, as Dave else has pointed, more complex solutions such as using an RDP session but you will need a "dummy" machine to run the RDP client that connects to your "hidden away" computer. You could use a small version of Linux running from a read-only USB - it won't really matter if the machine is turned off without properly shutting down - the next boot will simply read from the CD or USB. You could put a script that whent he machine boots it runs an RDP client in fullscreen to your Windows box. The Windows box stays on and safe, the Linux machine can be turned on and off as often as you like.