How do i eliminate a black rectangular block on my 25" zenith tv?

You have the closed-captioning turned on and set to "text" mode. It's easy to do on Zeniths because many of their remotes have on button just to control the captioning. Go to the caption settings and turn it off or set it to "caption" mode if you want to use it. Then the box will be gone.

(What's happening is that in text mode, the screen is blanked out in the area where the text would go, but there isn't any text being sent by the station. On channels where no captioning at all is used, the box never shows up.)