I agree that the barn manager needs to address this issue because clearly the horse is unsafe for any boarder that walks in there. What is he, or the horse's owner, going to do when someone goes in there to get there horse, and Joey kicks them in the head, sending them to the hospital? Who's going to be responsible and pay the bill?
If the owner doesn't know what his/her horse is doing, they need it brought to their attention.
As for yourself, I would try to not be alone when you go get your horse, in case something happens, someone is there to help.
It's very tricky dealing with an aggressive horse because if you show even the slightest hesitation or uncertainly, they could capitalize on it and call your bluff. That's the case where an agressive horse could become MORE aggressive with your trying to be the boss, if you don't "be the boss" absolutely perfectly. And that can be as little as you stepping backward from the horse, that can give them the cue to "attack".
So, make yourself as big and scary at all times, and LOUD! Don't be afraid to actually smack him with the whip if he needs it. Think of how a stallion would get another horse back in line if they misbehaved. They'd pin their ears in warning, they may squeal, and they certainly would nto have a problem biting or kicking the horse if they don't listen to their commands. Put those same principles to use. Have more of a forward stance with your body languange, and you can even sort of "crouch" (as if you were coiled like a snake ready to bite!). You want to appear as a predator figure to that horse with your body, to show you mean business, and don't mess with me.
With an aggressive situation like this, I wouldn't even be opposed to bringing something like a paintball gun to keep the horse away from me. At least with that, you aren't limited to the 5 feet of lunge whip. I don't take aggressive horses lightly. It's too dangerous.