User keeps on changing the invoice numbers of fully paid invoices by appending a "Z" before the invoice number using the Change Invoice # option. How to filter out all these "Z" invoices?
Because you are appending the "Z" to all your closed invoices (fully paid), you can get a list of these invoices by using the Open filter on the Invoices screen and selecting No for it. If you want to view only the open invoices, you can select Yes for that filter. Check the Core Help for more on filtering the Invoices screen. In general, if you want to get a list of invoices that contain the letter "Z", you could simply type "Z" in the search box on the screen.