Write a simple greeting such as "Dear Mr. Smith" or "Dear Ms. Smith" to open the letter.
Write a first paragraph that succinctly states a brief history between the addressee and yourself. For example, "This letter serves to confirm that you have contacted me regarding a possible working relationship in which you would hire my firm for advising services."
Continue this paragraph by listing out any papers or memos that may have passed between the two parties when any negotiations could have taken place. Note if any of that communication has been deleted or discarded.
Write a second paragraph that clearly states that you have not worked with the addressee and that no services or fees are due. This paragraph only needs to be a couple of sentences long. For example, "Because my firm has not provided services for your company no fees or dues are needed."
Write a final paragraph that clearly states your future intentions with the addressee. Some non-engagement letters are written to clarify the relationship between two parties, but when there is still a possibility of working together in the future. Another form of non-engagement letter is designed to refer the addressee to someone else for services. If you think you may want to work with the addressee in the future, state your openness to accept future communications. For instance, "I appreciate your interest in this firm. While we have not worked together in any capacity to date, feel free to contact me in future with possible questions or concerns." If you are writing a letter to refer the addressee elsewhere, clearly state that he should contact another firm or company with his inquiries.
Sign the letter with a simple "Regards" or "Sincerely," followed directly by your signed name underneath the closing statement.