The By Person worksheet is hidden in Excel Services, but people can see it if they open the file in Excel.
Here's an important note about hidden items.
Any items you hide using Excel Services Options will be hidden in Excel Services. That is, the items will be hidden from people when they look at the data using their Web browsers. However, the hidden items will still be visible if they open the file in the Excel program.
If you don't want people opening the file in the Excel program, be sure to review the information in Excel Services II: Requirements, recommendations, and permissions. That course explains how to limit certain people from opening the file.