Several options:
1) Disable the WYSIWYG editor.
2) If you must use the WYSIWYG, turn on the advanced toolbar (right-most icon in 2.3). That’ll show more buttons, one of which is “Paste from Word”. I believe that’ll strip out the extraneous formatting. If not, the “paste as text” button sure will.
3) Copy/paste to a plain text editor (like notepad) and then paste into WP