What would you do about a robin like this one?

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He sits outside my office window and attacks his reflection.  ALL DAY LONG!  It's been going on for about 10 days now, and it doesn't slow down.  We've tried all kinds of suggestions, hung up ribbon, opened the windows, and trimmed back the tree.  Tree trimming seemed to help for a day or two, but he never completely stopped.  Now he's back to hitting the window more than once a minute.  Without harming the bird, who seems to be defending his mate and nest, what can be done?  Please help.