You can control Flash cookies via Macromedia's Flash Player Settings Manager, which is an actual Flash application you load in your browser (that is, the picture you'll see on that page is not an image; it is the actual Settings Manager).
Set Global Storage Settings to 0, and then select the Website Privacy Settings tab (the fourth tab from the left), and select Delete All Sites to remove prior-stored history.
Note that the 'cookies' are (or were) stored in ~/Library » Preferences » Macromedia » Flash Player » #SharedObjects, and the settings are stored in ~/Library » Preferences » Macromedia » Flash Player » macromedia.com » support » flashplayer » sys.

