You can open the Sales Order (SO) and mark it as closed to purge these transactions after turning them into invoices. I'd be glad to show you how.
Are you referring to the sales orders on your Sales Order Fulfillment Worksheet or the Open Sales Orders by Customer report? You can double-click the SOs to open them and mark them as closed to prevent them from appearing in your reports. Here's how:
1. Hover through the Sales Order on your reports and double-click to open it.

2. Click the Mark as Closed button.
3. Select Save and Close.

I'm adding these articles with the topics you can use while recording the common vendor and customer transactions:
By following these steps, you should be able to purge your sales orders that have been turned into invoices in QuickBooks Desktop. If you have more questions about your sales orders in QBDT, don't hesitate to send a reply below. I'd be sure to answer them for you at any time possible. Have a great day!