I agree Wendi - this would not in any way be considered spam.  You are faxing them an unpaid invoice for a service they ordered not soliciting business you don't want.  I don't think it is unethical either - just make sure you don't want any future business arrangement with this company - it would be irritating but at the same time so are ignored phone calls and past due notices.  The repeated fax would certainly draw attention to your company and if it worked you would lose less value of the money owed to you.  While I have always tried to maintain a level of not only business maturity but personal maturity in dealing with past due invoices, maybe lowering yourself or your company to the standards of the company you are dealing with is the best way to deal with the company.  Like I have said before, sometimes it is like any other relationship - did you ever find out that sometimes the most effective way of getting across to them is getting down to their level of understanding.