Brian's suggestion for pasting new content in the plain text clipboard will help prevent the need of removing text formatting in the future. If you're using Google Chrome, you can also right click and choose Paste and Match Style. I've found this is useful when pasting into the Knowledge Base or ticket responses as this will remove any current formatting of the text in your clipboard and use what's in the message body.
To remove current formatting in the message body you can use the Remove Format button from the text editor. To access this button you'll need to have your editor option expanded. The icon is an underlined "T" with a small "x" to its right. Just highlight the desired text and click the Remove Format button. While this doesn't make the text plain text, it'll strip out the style, span, etc. attributes.
I hope this helps!