Last Updated Oct 24, 2010 11:33 PM EDT
Before I get to that, it's worth pointing out that you can change pasted text at the moment you press Ctrl+V. When you paste text, press the Ctrl key a second time (or click the Ctrl button on screen near the pasted text) and choose Keep Text Only.
You don't necessarily want to do that every time, though, so click File, Options, and then the Advanced Tab. In the Cut, copy, and paste section, you can specify the default you want for all sorts of situations, such as copy/paste from within the same document when pasting text from another program.
That's it -- after you make the change, you won't be annoyed by unexpected formatting changes ever again. [via How-To Geek]