Purchasing has a task, "Close Orders".  You can read more about this in Purchasing - Help - Help Topics.  Using the index tab, type "close" in the search box for help.  It will explain what it does and how to do it.  It won't close an order with a pending change order.  I will tell you that Change Orders themselves can be a problem so we don't really like to use this method.  However, if you aren't expecting these older orders to ever be received, you should probably close them.   There are also year-end procedures for Purchasing that allows you to clean-up your files so older information doesn't populate on most of the reports.