I don't think you really want him to intimidate repairmen. If he does, then he's being encouraged to intimidate strangers that are invited inside. This could be anyone, including your extended family, or a new friend. I always crated my pup and only let him out after I knew for sure that this new person actually liked dogs. And of course - once my pup was quiet, lol. But this won't changed how people knocking at the door are received - it's just that once they're invited in, I think manners are in order.