How can I prevent notes other than those belonging to the scale from being played in Ableton Live 12?

There are a few ways to restrict notes to a specific scale in Ableton Live :

1. Use the global Scale feature to set a scale for the entire Live Set. This will make MIDI devices like the Arpeggiator and Chord always generate notes that fit the selected scale. You can also “lock” certain MIDI clips to the global scale.
2. Use the Scale MIDI effect on a MIDI track. This allows you to map incoming MIDI notes to a specific scale. Notes outside the scale will be transposed to the nearest note inside the scale.
3. In the piano roll MIDI editor, you can select notes and use the new “Fit to Scale” transform to automatically move the notes to fit the current clip scale.

So in summary, the Scale MIDI effect, piano roll transforms are the main built-in ways to restrict notes to a scale in Ableton Live 12. The new global Scale feature also makes it easier to keep everything in key across your Live Set.