No need for Better Privacy, use this
You don't need the Better Privacy plugin. Just browse to the Macromedia folder on your machine (it's usually somewhere in Application data, search for it), delete the #SharedObjects folder and then create an empty file called #SharedObjects, without extension. That way Flash will never be able to create a new directory to store its Flash cookies. Works perfectly.
You could also take this one step further and create an empty "Macromedia" file in Application Data.
Problem solved for good.