I would like to keep a single product listed on the store to show that it is now obsolete, but I need to stop it being backordered by a customer.
Unfortunately Cloud Commerce has no way to choose which products can be backordered - it's all or nothing. However, it's possible to modify the front-end templates so you can choose to hide the Add To Cart / Backorder button on discontinued products. Here's an example of how it can look:
It's a common request but a bit technical to pull off, so you'll need the assistance of a Maropost Partner:
https://maropost.partnerpage.io/partner-directory?Maropost Product=commerce