It's nice to see you in the Community today. I can assist you with inactivating your customers who don't have any activities in your QuickBooks Online.
Pulling up a report that shows customers without any transactions in QuickBooks Online is not an option to us. However, you can pull up a Transaction List By Customer report to help get the information you need. The customers that are not included in that report indicate they have no associated transactions. You can export the report into Excel and go through each name and compare it to your customer's list.
Once done, you can start inactivating them. Here's how:
Click on Sales at the left pane.
Select on Customers.
Locate the name of the customer that is not included in the Transaction List By Customer report.
In the Action section, click on the drop-down arrow and select Make Inactive.
I'd also encourage you sharing this idea to our Product Developers. Doing this helps them understand where we can improve and what features should be added so we can better serve you. You can do this by clicking on the Gear icon then select on Send Feedback Online. Your willingness to help us improve our product is much appreciated.
Please let me know if you need further assistance with your QuickBooks account. I'm here to help you out whenever you needed me. Have a great day!