Word can be tricky to work with. Yes, it's an option in the WYSIWYG editor (and we're all about giving our users options!), but not one we recommend. You can save yourself some trouble by using 'paste as plain text' and then do any necessary formatting, such as bolding, indenting, or changing the font size.
If you're replacing/updating an existing page that had text on it which you know came from Word, click on the <> button in the tool bar - you'll see the underlying code. Highlight everything and delete it. Then click <> and use 'paste as plain text' to post your new content.