Hm. If you don't want to restrict access to the room, and don't want to restrict access to power, you'll have to restrict access to the machines themselves. You could apply a padlock to a moving part of the device (or perhaps a cage around its controls). You'd then store the keys in a central place and require people to sign out those keys when they want to use the device.
If you don't want to rely on people's good faith in signing things out, you could have someone else control the keys. This would require some manual work, but it could probably be done by a department secretary or someone else who's already at a desk - you wouldn't have to put someone in the room itself, and it would presumably be a very small part of their job.
You could conceivably store the keys in some sort of container with electronic access control as well, but that may be more trouble than it's worth.