Limit plex access

Is there any way to limit plex so it will only allow access from certain machines? I want to be able to give someone the username and password to login as needed, but only from one specific computer, so they can’tjust install it on every devic they own.

This is unfortunately not possible.
If you’re a networking expert you might be able to fashion something out of a custom router firmware, but there are no functions within Plex to support this.

The best you can do is to:
have the user create his own plex.tv account and share your server with that. So your own plex credentials don’t leave your hands.

Install PlexPy and let it collect usage data. If you recognise concurrent playbacks from more than one clients with his user account, warn him. It it continues, stop sharing with him.