If you edit a character (press the edit button on the menu that appears when clicking on a character in the right side bar), you can add things to the 'in players journals' and 'controlled by' boxes. Conveniently, you can just click on a name to add that person to the 'list'.
However, removing items from those boxes is not so easy. Clicking on the names already existent doesn't do anything. It's possible to click into the square, try typing something, then press 'backspace' until you've removed all items, but you can't select a single item and then delete it (backspace or delete key), mouse clicks have no effect except to allow you to enter text into the box.