BuddyPress.org

Try this:

function remove_wysiwyg($enabled, $field_id) {
    // change this field_id to the one you need to have ordinary textarea
    if ( $field_id == 22 ) 
        return false;

    return $enabled;
}

add_filter( 'bp_xprofile_is_richtext_enabled_for_field', 'remove_wysiwyg', 10, 2 );

Place the code in bp-custom.php.

if ( $field_id == 22 ) { of course. Shame on me.

Glad it works for you.

I’ve added the mentioned code in my bp-custom.php. I get a 500-er error.
I just changed the field_id, nothing else. Any ideas?

WP: 4.4.2
BP: 2.5.1