It happens to the best; sometimes you get so overly excited downloading demos, games and free content from the PlayStation Store, that you accidentally download something you didn't want in the first place. Fortunately, it's not too late. Stopping the download before it is finished means you won't have to go through the trouble of deleting it from your console after it installs. Stopping a download only takes a few seconds and is readily done by anyone old enough to work a PS3 controller.
Press "Left" or "Right" on the controller to scroll over to the Network section on the XMB (cross media bar), also known as the PS3's Home menu.
Press "Up" or "Down" on the controller to scroll to Download Management, an option that appears only when you are actively downloading something to your PS3.
Press "X" to select Download Management. Here you will see a list of active downloads.
Press "Up" or "Down" to highlight the download you want to cancel. Select it by pressing the "Triangle" button, choose "Cancel," and press "X." This interrupts the download and removes it from your console.