In Outlook 2007 that was possible with VBA:
Private Sub Application_FolderContextMenuDisplay(ByVal CommandBar As Office.CommandBar, ByVal Folder As Folder)
CommandBar.FindControl(, 2500).Delete True
End Sub
In Outlook 2010 the Office.Commandbar still exists but should not be used anymore. The example above doesn't work anymore, at least not on my machine. Test it, maybe it still works for you.