Delete from Work Menu
Recommended Method
Use the free AppleScript from "melisofao" that lets you delete items from your Work Menu. It offers up a small dialog that lists all your Work Menu items. The script may be downloaded from Scriptbuilders.net [Lene Fredborg, 19-Jun-2018: Removed outdated link to http://scriptbuilders.net/files/removework1.11.html] and melisofao reports that it works in Word 2004 and Word 2008. (If this link is broken, search Scriptbuilders for "Remove Work".)
Save the script into [username]/Documents/Microsoft User Data/Word Script Menu Items.
If you don't want to deal with scripts, you can use Method Two, but it has risks.
Alternate Approach
Word has a built-in command named ToolsCustomizeRemoveMenuShortcut. To remove items from the Work menu, you can make this command easily accessible by putting it on a menu or assigning a keyboard shortcut to it.
ALERT! This command will remove ANY entry from ANY menu. After you select it, it will let you drop down a File, Edit, Work, etc., menu, but then the next menu entry you click on will be GONE. Be careful. ESC does not cancel it.
You may wish to use Help>Send Feedback in Word to ask MS for an easier, more accessible, and less risky method (or visit the feedback website [Lene Fredborg, 11-Feb-2018: Removed outdated link to http://www.microsoft.com/Mac/suggestions.mspx]).
Add Keyboard Shortcut
There is no longer a default key combination because Apple added a system default shortcut that overrode the old default. You can create your own.
Bring up the Tools>Customize Keyboard dialog. Hit Commands, and select All Categories in the left column. Tab to the right column.
Quickly type "too" to jump you go the Tools section of the right column, and find the ToolsCustomizeRemoveMenuShortcut.
Enter and assign a keyboard shortcut.
Add Toolbar Entry
Bring up the Customize Toolbars dialog.
Word 2008 and later: use View>Customize Toolbars and Menus or right-click a built-in toolbar.
Word 2004 and earlier: use Tools>Customize Toolbars and Menus.
In the Customize Toolbars dialog, hit Commands, and select All Categories in the left column. Tab to the right column.
Quickly type "too" to jump to the Tools section of the right column, and find the ToolsCustomizeRemoveMenuShortcut.
Drag it to the fake "Work" menu that appeared when you brought up the Customize Toolbars dialog—hover to open the Work menu, then release. You can then right-click it and select Properties to rename, if you like. (You don't have to put it on the Work menu—you can drag it to any toolbar or menu location you please.)
Hit OK to exit out of everything. Now you have the ToolsCustomizeRemoveMenuShortcut command on the Work menu.
From the Work menu, select the ToolsCustomizeRemoveMenuShortcut entry, and then go back to the Work menu to delete the file.