For devices located in Common Areas it is recommended to deploy a Lync Common Area Phone account; see this article for detailed instructions:
http://blog.schertz.name/2011/04/common-area-phone-configuration-in-lync/
You cannot prevent Lync users from signing-in to the device, but a quick sign-out or reboot of the phone will return it to the Common Area account by default.
Jeff Schertz | Microsoft Solutions Architect - Polycom | Lync MVP