When trying to edit metadata, I get an error "This property cannot be modified."

I took some photos with my iPhone 4, copied them to my desktop, and now I want to remove the GPS metadata from a few of them.

I tried to remove the properties directly in Windows 7,  but I get an error saying "An error occurred when writing the property 'Latitude' to the file 'filename'."

So I decided to try it in Bridge CS5. I selected the photo, went to the metadata panel, double clicked the location under Latitude, and I get an error saying "This property cannot be modified."

Thank you for your help!