Simple way, uncheck the option to display recently used files. Tools =>
Options => General (tab). Close and reopen Word. Check it again and change
the number back if you originally had a number other than 4.

More dangerous method: Ctrl-Alt-- changes the mouse pointer to a big minus
sign. (That is the keyboard dash, not the numeric keyboard minus sign.) Use
it to pull down the file menu and click on the offending document. It will
remove it. It will also remove any of your other commands!