How to disconnect Apple TV from iPhone's remote control?

The software remote in Control Center does discovery in the moment that you bring the widget to active, and then lets you select which Apple TV box to connect with, if there are more than one. No reason to ‘disconnect’ anything, nor is that possible.

Or do you perhaps mean the Now Playing tile on the iPhone Lock Screen?

While the iPhone it’s in the same WiFi network as the Apple TV too the remote control will be there in control center , you can’t delete it