I can't give you a definitive answer simply because I don't have multiple user accounts to test on currently. Also you did not specify which version of Word.
However, assuming Word 2007, which I have just taken a look at:
Go to the 'review' tab and then click the button on the far right labelled 'protect document' to the far right. I note you can apply editing restrictions to particular users. What I couldn't test without another user account to use is whether you can change these restriction settings with another account.
Also on this screen there is a 'restrict permission' link in the bottom right which I believe would be much more comprehensive, but this requires a further installation from Microsoft it seems.