When working in a web editor, you need to be sure to copy and paste in data WITHOUT any hidden formatting from an application (e.g. MS Word). You even need to be careful when copying from your browser, since even HTML can get added to the clipboard.
One (not so effective) method is to paste everything into another application such as NotePad (Win) or TextEdit (Mac) … and then copying and pasting the text from there. Essentially, doubling your work.
There are however, many apps that can handle this for you…and there are very good free options.
For Windows, PureText.
For Mac, Jumpcut.