Action Hook: admin_print_styles-profile.php

It’s very easy to override the user-selected rich edit setting. Whether they’ve selected to show it or hide it, it can be reversed in a single line of code:

Always show:

add_filter( 'user_can_richedit', '__return_true' );

Always hide:

add_filter( 'user_can_richedit', '__return_false' );

Of course, if you’re going to overwrite this setting, it would make sense to also hide the user’s checkbox for this option.

add_action( 'admin_print_styles-profile.php', 'hide_rich_edit_option' );
add_action( 'admin_print_styles-user-edit.php', 'hide_rich_edit_option' );
function hide_rich_edit_option() {
    ?><style type="text/css">
    label[for=rich_editing] input { display: none; }
    label[for=rich_editing]:before { content: 'This option has been disabled (Formerly: ' }
    label[for=rich_editing]:after { content: ')'; }
    </style><?php
}

You can probably remove it more fully with Javascript, but I’ll leave that up to you.