I just found myself doing one of these so I thought I'd post what we use to communicate with non-clients the fact that we're not representing them. You do not want to unwittingly create an attorney-client relationship when you don't actually want to represent someone. Also, having just renewed a malpractice policy I recall that the use of non-engagement letters is one of the questions on a malpractice application and not using them likely impacts your premium for the worse.
Here's our non-engagement letter:
Thank you for your time and consideration. I have appreciated the opportunity to talk with you and discuss our firm’s potential representation of you regarding various pending domestic relations related matters. I have reviewed the information you provided me, but for various reasons, we have decided not to represent you in these matters.
Please email or call if there are questions or if there are additional legal needs more suited to our practice areas. In declining, we express no opinion of the merit of your case or the potential for its success or failure. You should seek legal assistance from another attorney.
Please note there is a limited amount of time for you to pursue this matter. If you are to proceed, you should do so with haste. Time could be critically short. I recommend that you immediately contact another attorney for assistance. The Chicago Bar Association has a lawyer referral service and can be contacted at 312-554-2001.
In declining, we express no opinion of the merit of your case or the potential for its success or failure. You should seek legal assistance from another attorney.