With this filter you can control who can reply to the thread dynamically and you can also left custom message if needed.
add_filter( 'bp_better_messages_can_send_message', function($allowed, $user_id, $thread_id){
$is_restricted = true;
if( $is_restricted ) {
$allowed = false;
/**
* With this global variable you can add extra message which will replace editor field
*/
global $bp_better_messages_restrict_send_message;
$bp_better_messages_restrict_send_message['your_unique_error_slug'] = 'You cant reply to this thread';
}
return $allowed;
}, 10, 3);
This is the example of blocked thread:
