Is there a way to trully disable AI features, and enforce it across all settings, users?

AFAIK, as of now, there isn't a direct way to disable the Automatic Assistant Autocomplete features for all users at the account level.

Individual user has control to disable Automatic Assistant Autocomplete they need to disable the feature themselves in their developer settings.

As per document to disable the Automatic Assistant Autocomplete you need to do it from user settings.

• To turn off autocomplete suggestions, toggle Autocomplete as you type. When autocomplete is off, you can display autocomplete suggestions by pressing Ctrl + Space.
• To prevent Enter from inserting autocomplete suggestions, toggle Enter key accepts autocomplete suggestions.