Items will be removed from the "Recently Watched" row on the "For You" page if they have been watched to completion.
Based on user feedback, Comcast will be adding the ability to manually remove titles from Recently Watched in a future release.
Items watched all the way through do not automatically clear. The time remaining simply resets to the time of the entire show. It would be nice if there were a way to clear them. It's just clutter, especially when a family shares the account.