Schedule Shutdown w/ Network Connection

I have two iMacs that use files from each other.

I want to schedule them to shut down each evening, but if connected on the "network", they won't shut down, until I click the button on this error message window:

"There is a user sharing this computer. If you shut down, they will be disconnected. Click Shut Down to disconnect the user and shut down the computer."

How can I force it to do so automatically (or to disconnect from the network automatically if idle for a specified amount of time)?

Thanks!

-Aaron