Something that you could maybe do, is use the Credit Hold option on the customer record. Manually set it to "On". You'll need to make sure that under Setup>Accounting>Accounting Preferences that the "Enforce Holds" option is on, and that "Include Orders" is checked. Doing this will make it so that you cannot create sales orders or invoices for that customer. You will still be able to create Opportunities for the Customer still, but no sales orders or invoices.