If you don't want to disable Flash content, but just to make a disturbing Flash ad shut up, you can install FlashMute, a small program that supports IE, Firefox, Opera.
When executed, FlashMute will put a new icon in your system tray. When clicking this icon, it will alternate between its normal state: "F", and the muted state: "F" with a red X across it. When in the normal state, Flash movies will be allowed to use sound output, but when the red X is displayed, Flash will be denied access to your audio hardware.
You can also use a shortcut instead of clicking the icon: Ctrl-Alt-M.