"Good 'Til Canceled" listings are the default type of listing that JoeLister creates on eBay (the other choices are 30 days, 7 days). With these listings, when the quantity reaches 0, JoeLister does not take the listing down.
This is done so that the listing's sales history is preserved, and its better for SEO, too. When you restock the item, your listing will retain the search-ability strength that it has built up over time. If you no longer plan on restocking the item, you should "Delist" the item through Joelister's 'eBay Listings' tab.
The 'eBay Listings' tab is the best way to see how many listings are still lingering. Sorting by ascending order will make the 0 quantity items come up first, so you can find and delist them more easily.