At this time of year, birds become territorial. The robin is seeing its reflection in the window and is trying to "drive away" the "intruder". This behavior can be irritating for the human occupants, and potentially harmful to the bird as it can severely injure itself by flying into the window.
I'm guessing he tried hanging the garbage bags on the inside. But that won't get rid of the reflection on the outside. Try hanging the garbage bags on the outside of the doors. Or you can use soap to temporarily smear the windows so that the bird cannot see its reflection. Whatever you use, it needs to be on the exterior not the interior of the glass because it's the reflection that's the problem.
Cornell has more info. about this behavior and solutions. Go to the website below and scroll 1/2 way down the page. http://www.allaboutbirds.org/Page.aspx?pid=1184