Bruce Schneier explains:
"Unlike traditional browser cookies, Flash cookies are relatively unknown to web users, and they are not controlled through the cookie privacy controls in a browser. That means even if a user thinks they have cleared their computer of tracking objects, they most likely have not.
What’s even sneakier?For detailed information see The Electronic Privacy Information Center web page on the subject at:
Several services even use the surreptitious data storage to reinstate traditional cookies that a user deleted, which is called re-spawning in homage to video games where zombies come back to life even after being "killed," the report found. So even if a user gets rid of a website’s tracking cookie, that cookie’s unique ID will be assigned back to a new cookie again using the Flash data as the "backup.""
I have been testing a program called, humourously enough, Cookinator. Seems to be doing a lot of cleaning when I use it and so far have not had any resulting system problems.
Information on the free free Cookienator program can be found at: http://codefromthe70s.org/cookienator.aspx
(January, 2011 note - we have had excellent success with the C-Cleaner program for this problem)