Just delete the original document from wherever you have it saved, it is only listed as you have recently opened it.
If you just want to delete the history or change the amount of Docs displayed. Try this.
Open office 2007 click on Office button, then Word Options.
Then the advanced tab, scroll down to the Display section, then Show this number of Recent Documents. Setting this number to 0 will clear the list. You can then reset the number if/as desired.
Remember Brian' Google is your friend.