StopTaunting is a simple addon that displays a message when your current target has a successful cast of Taunt, Growl, or Mocking Blow used on it. Currently, StopTaunting only displays this message in the chat window, but future versions will include an optional message in the middle of the screen.
Slash commands: /st msg <message to display>
Future versions will include options to turn off either of the display methods.
Please report any bugs that you find. I didn't have much time to test this, honestly, so I'm not sure that it works 100% quite yet.