If it's private property as it says, seems there's not a lot that can be done. If it's on city property or a right-of way, different matter. Check the back of the sign and if says that the sign is city/county/state property I would question why public funds are used for a sign on private property.
Edit: I just noticed you are in D.C. I lived in D.C/ Maryland for several years. Your location explains a lot.