Scroll down to the Voice Commands section and check "Disable built-in voice commands" checkbox
Option 2: Disable individual voice commands
You can use the <delete> command to delete individual built-in voice commands.
For example, say you want to delete the built-in voice command question mark => ?, i.e., when you say question mark, you don't want it to insert ?. You can add a custom voice command question mark => <delete> . This will remove the built-in voice command.
Refer to the Custom Voice Commands guide below to learn how to add a custom voice command.