zabotlama's answer is probably the best by letting the customer know up front, that your models are not suitable for 3D printing. Most people interested in 3D printing just assume that if it looks good on the screen, it will print fine, which is not the case. "Suitable" may be a somewhat vague word, because it involves an opinion. Perhaps even go a step further, saying that your models are NOT 3D printable and hopefully then, you will get less questions of "Can you make this 3D printable?".
You do some fine work, by the way. Keep it up.