I had the same issue but, after some playing around, noticed that you can't do it while the Smart screens are showing. I had to go back to the TV viewing and then into Menu -> Smart Features and now 'Clear viewing history' is enabled. I guess it can't clear it while it's loaded for some reason.
On my samsung 40"led series 5500 tv, i cannot delete the "enjoyed content" on the photos,videos and music part of the smart hub. When I go to menu , then smart features then "remove viewing history" it is not lit up and when i click on it is says that this function is not an option! is there another way the delete viewing history?

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