Fencing the table: Excommunication judgmental or act of mercy?

I'm having a conversation with a liberal Lutheran about the issue of fencing the table.  His argument is that refusing someone communion because of unrepentant sin is a slippery slope.  That's a logical fallacy and I'm not going to engage it. 

But what I do want to dispel is the myth that a priest doesn't have a rite to judge.  First there's St. Paul...

"Do you not know that we will judge angels? How much more the things of this life!"

That was written in the context of the excommunication issue.  Like he already anticipated the liberal Lutheran perspective. 

But then there's Jesus...

"If you forgive anyone's sins, their sins are forgiven; if you do not forgive them, they are not forgiven."

None other than Jesus Himself has given the church, through the apostolic ministry of the 12 and their successors, the authority to judge these things. 

I have every right as a priest, with my bishop's approval, to bar an unrepentant sinner from the table and I'm going to do it, because that's the right pastoral thing to do.  Both to protect the sinner's immortal soul and the sanctity of the eucharistic meal. 

Part of the problem here is the protestant denial of sacramental grace.  Priests can't forgive sins in that system, "because only God can forgive sins."  Now if only God can forgive sins, then only God can judge sins.  But that's just not how God set it up.  I guess he could have, but He didn't.  

Instead He gives objective means of grace and sacramental ministers to administer them.  Confession is a sacrament of the church, when one goes to his priest with a penitent heart and makes his humble confession, its not the priest forgiving him, but rather Christ through the priest.  I think that makes a priest in a unique position to be able to guard the table and his flocks soul.  

Now of course this looks a certain way on the outside looking in.  If you don't understand sacramental theology then it would make sense that you also wouldn't understand the priest's responsibilities at the altar.  Instead pastoral care looks like angry judgment, but its not and the fact of the matter is that any controversy that follows is simply a matter of ignorance and often poor catechisis.  Excommunication, therefore, is crucial to the church and is one of the pastor/priest's greatest tools in caring for his flock.