In OS X 10.9 Mavericks, Apple introduced some new functionality to multi-screen setups. For normal day-to-day users, these new features are helpful, however, if you're running ProPresenter or ProVideoPlayer, these changes are troublesome.
One of the changes Apple made is showing the menu bar and Dock on the second screen. Obviously you don't want either of these showing up on the displays your audience will be seeing.
To disable this feature, open System Preferences and click on Mission Control on the first row of options. Uncheck the Displays have separate Spaces options. You will have to log out and then log back in for the change to take affect. After making this change, the menu bar and Dock should not be visible on your secondary displays.