I suppose if I were in your situation I would bug the realtors like crazy. Everytime you get bugged by it give them a call until you can talk to a live person. Maybe then you could give them a time limit and tell them that you will have it disposed of properly.
It's funny that a few of you mentioned Girl Scouts. I was a Girl Scout (my mom was my troup leader), and it's the Girl Scout inside of me that really wants to go over there right now and get that thing down. I guess I'll give the realtor a few days, but then I'm taking it. I'm kind of afraid that if the realtor actually does do anything with it, he'll just throw it away.
I'm actually the VP of our Home Owner's Association, so I guess I could take it down in that role. We have very strict rules about the appearance of houses, and ripped up flags are definitely not allowed.