I'm here to help share information about flagging customers that are no longer in a good standing with your company.
As of now, there isn't an availability to flag customers in QuickBooks, however, I have a workaround for you so you'll be able to identify those customers. If you're using QuickBooks Online (QBO), you may create a parent customer and add those customers that are not on the good standing as a sub-account to that customer.
On the other hand, if you're using QuickBooks Desktop (QBDT), you must drag the asterisk icon beside the customer name to make it as a sub-customer.
You can see attached screenshots for additional reference.
In QuickBooks Desktop
Please let me know if you have any other QuickBooks or customer questions, I'll be always here to help you. Have a great day ahead!