Role to role block is applicable only when sending direct message. Eg. if you block A to B role then A will not be able to send direct message to B.
It is not applicable when sending group or admin message.
Please go to class-fep-group-message.php line 320 and change return $user_groups; to return apply_filters( 'fep_get_user_groups', $user_groups, $user_id ); (this hook will be added to next version, so you will not be needed this change for next version)
Then add following code in your theme’s (child theme’s if any) functions.php