Vista - Cannot stop file sharing on a directory

My documents folder appears to have become shared with another user on my
computer. I want to turn this off. Alternatively I would like to turn it off
for a sub folder of my documents. I am using Vista Home Premium.

When I select Share on the directory, and then try to stop file sharing or
remove the other user from being allowed to share, the system thinks about it
for a while and then comes back with "You could not stop sharing the selected
folder" "click here for more info ...". It then says "Some items are still
shared" and "this folder is a shared folder"