A few more steps...
Block OTA profile (also tvOS 13) has started downloading.
Make sure you open BetaProfiles.com in Safari browser.
You will see the following prompt:
Tap Allow from the popup
Then follow the prompts to install tvOS 12 beta profile in Settings app and restart your device.
After your device has restarted, you will no longer receive any new iOS updates.
Make sure you have deleted the downloaded update in Settings - General - [Device] Storage.