Can I post on my website that a business man has not paid me for services rendered, i.e. writing a business plan for him?

The first thing that will need addressing is the "statute of frauds" requiring a writing to be enforcable. Even if the debt is legitimate posting it on your website is an invitation to liability, that is what the credit reporting companies do and in addition, that proably will not get the debt paid. I would suggest you look into small claims in the jurisdiction where the debtor lives or turn it over to a collection agency and deduct the amount on your taxes.

Good Luck

The second part of your question is really in the realm of defamation (aka libel/slander). You are asking what you rights are to say things about adverse parties in a public space without repercussion. In the world of defamation, generally across all states (although I am not licenses in WA), truth is always a defense. This means that you can always say something that is true. However, that does not mean that this ex-client may not try some form of legal action against you for aleged libel. Nonetheless, it should be fairly easy to prove that he, in fact, did not pay, and you will have to weigh the risk of recourse against you against the "truth" of your proposed statements.

I hope this helps.

Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.