how to unauthenticate my desktop?

Unauthenticate from what service or feature or mechanism? From the iTunes Store? If you're looking to deauthorize from iTunes, launch iTunes.app and select Store > Deauthorize.

Apple has available a recommended sequence for preparing to sell or give away a Mac, if that's what you're doing here.

Apple Remote Desktop (this forum) does not have an unauthenticate sequence, the close command or other disconnection will disconnect. If you want to prevent access via Apple Remote Desktop or control some of the related settings, you can manage that via System Preferences > Sharing.

If there's a saved password around that's involved in whatever you're seeking to unauthenticate, remove that from your keychain using the Keychain Access.app tool.