Inactive Client Category

It would be a nice feature to be able to mark a Client as Inactive. I have users that were created for clients because I have invoices for them but we are no longer doing business with each other. I would like to delete their user to limit unnecessary points of entry to the website but it would effect their invoices. Even if I could delete them, suppose I do business with them again a year later? If I could just make them Inactive so they wouldn’t have a login option unless they were an Active client.