Flash has the equivalent of cookies, and they’re hard to delete:

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?

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.”