Disable Parent Customers from Invoice

Hello,

I would like to block parent customers form being able to be chosen in the Sales Order/Invoice.

So in the Sales Order form I can only choose sub-customers only and not the parent customers.

More Info:

In our current system we do not use locations for customers because we need each outlet/location to be sub-customer or child of the main customer.

So for example Walmart will be the parent customer and the different stores each will be a child.

However we want the parent customer to never be invoiced directly because it isn't a specific store.

We want the parent customer to be used to be an umbrella for the different stores in our reporting.