HowTo: Clean up text in WordPress posts
If you find you have a WordPress post that has text that doesn’t match the styling on the rest of your site, more than likely that post has gotten inserted with some extra CSS styling. This usually happens when you copy and paste from a word processor without using the “Paste from Word” button in the WordPress Visual editor toolbar.
My best method for cleaning up this problem involves cutting the text from the post via the Visual editor, pasting it into Notepad and then re-pasting the text back into your post via the HTML editor. To see how this is done, watch this screencast.