If you start new mail threads based on replies (which can be a real timesaver in and of itself), you'll probably run into the occasional issue with undesired formatting. Even if you clear out the body of the message entirely, your text may be the wrong style or indented due to a phantom background image.
The quick way to clear this is to toggle your message format. On the message window, go to the Format Text tab and click Plain Text:
If there is any formatting to clear, you'll see this message:
Just click Continue to clear all formatting. Then, if you want to switch the format back to HTML or Rich Text, click the appropriate button and start typing your message.