Can't deactivate customer

Hello markdrury, 

Welcome back to the Community. I trust your business is going well on your end. I'll be glad to show you how to make a customer inactive in QuickBooks. The program is designed to help you get ahead with your work using the versatile features offered in the program.

In order to make a customer inactive, click on Sales from the left navigation menu > click on Customers > from here, locate the customer in question > click the drop-down arrow and select Make inactive. Here's a screenshot of the drop-down arrow I'm referring to:

Give this a try and let me know how you make out. I'll be a message away in case you need anything else.