Turn off Apple Intelligence in macOS 27

Siri

1. You will need to enable Screen Time to block Siri. Go to System Settings > Screen Time > Contents & Privacy.
2. Turn off the Contents & Privacy switch at the top of the screen.
3. Click Siri.
4. Change Siri to Siri Classic.
5. Go through the list of Capabilities and turn each one off.

Notifications

1. Go to System Settings > Notifications.
2. Turn off the Summarize notifications.

Messages

1. Launch Messages.
2. Go to Settings > General.
3. Turn off Summarize Messages.

Mail

1. Launch Mail.
2. Go to Settings > Viewing.
3. Turn off Summarize Message Previews.
4. Go to Settings > Composing.
5. Turn off Personalize Smart Replies.

Journal

1. Launch Journal.
2. Go to Settings > General.
3. Turn off Get Writing Prompts.

See: Turn off and restrict access to Apple Intelligence features on Mac - Apple Support

Siri & other apps

1. Go to System Settings > Siri.
2. Click on App Access.
3. Click on each app and…
4. Turn off Learn from this application.
5. Turn off Show Siri Suggestions in application, if shown.
6. Repeat for each app in the list, there should be at least 70 of them.

See: Customize Siri on Mac - Apple Support

Note: In System Settings > Privacy & Security, there is an Apple Intelligence Report. I recommend keeping this setting turned on so that you can verify that Apple Intelligence is truly not functioning.

Also review Analytics & Improvements and Apple Advertising to ensure you know what information is being sent back to Apple and other companies.