Apple TV
How do I disconnect Apple Tv from my iMac 2017?
iMac Line (2012 and Later)
Apple TV connects to a Mac using home sharing or airplay.
For airplay you would just click on the airplay logo within the video and stop the stream, or disconnect from mirroring.
How to AirPlay video and mirror your device's screen - Apple Support
For home sharing, the connection will cease when the Mac is asleep or off. If you want to permanently disconnect then you can go into iTunes and disconnect the library, as well as via iTunes settings.
Use Home Sharing to share iTunes content with other devices - Apple Support