With SD programming, the Closed captions data is kept on the top line or two. Usually a TV has its Overscan set in a way that these top lines are usually off the screen and out of view. In other words, you aren't meant to see them. That worked fine in the CRT days, but with Plasmas we seem to be able to seem them more easily.
Just go to your Plama's set up and see if you can stretch the height of your picture until the lines are out of view. Otherwise, just learn to accept them.