Add a new option to sub settings, to require users to flair their posts, in addition to the option that permits users to flair their posts.
If either of those options are set, show the sub's list of flairs in the Submit a Post form, and allow or require the user to choose one, depending on the sub settings. Mods are not required to use flair in their own sub, even if they have set the require flair option.
If users are required to flair their posts, don't allow them to remove the flair from their posts, but do allow them to change it. Mods are still allowed to remove flair from any post in their sub.
Add a new option to sub settings, to require users to flair their posts, in addition to the option that permits users to flair their posts.
If either of those options are set, show the sub's list of flairs in the Submit a Post form, and allow or require the user to choose one, depending on the sub settings. Mods are not required to use flair in their own sub, even if they have set the require flair option.
If users are required to flair their posts, don't allow them to remove the flair from their posts, but do allow them to change it. Mods are still allowed to remove flair from any post in their sub.