Killing the Word process outright is possible (see some of the other replies), but outright rude and dangerous: what if the user has important unsaved changes in an open document? Not to mention the stale temporary files this will leave behind...
This is probably as far as you can go in this regard (VB.NET):
Dim proc = Process.GetProcessesByName("winword")
For i As Integer = 0 To proc.Count - 1
proc(i).CloseMainWindow()
Next i
This will close all open Word windows in an orderly fashion (prompting the user to save his/her work if applicable). Of course, the user can always click 'Cancel' in this scenario, so you should be able to handle this case as well (preferably by putting up a "please close all Word instances, otherwise we can't continue" dialog...)