If they set the attribute on the Everyone group, then it will be the default option for every user unless that user had specifically went in to where those options are, left the option disabled, and saved settings there themselves, therefore creating their own user specific attribute for that setting. The user attribute would override the Everyone attribute. As long as there is no user attribute already set for that option, then they're fine. If there does exist a user attribute for it, then that will need to be deleted so the Everyone attribute is used.

If you're asking about a method of making sure that the user doesn't have a specific attribute for that option or that it's set to Yes instead of No, and they don't want to manually do this by going into user properties individually, then you can perhaps use the SDK to programmatically do this.