Online dating site etiquette; how do you end it?

If someone has exchanged a few messages with you and you can see s/he is not for you, what should you do? I can see three choices; a) Just stop answering, b) Tell the person a white lie and c) Tell them they are not for you. If you tell them they are not for you, would you tell them why? I have felt lousy when someone just doesn’t answer an initial message and am wondering about the “etiquette” of that as well.

Also, how long would you engage with someone if it is likely, but not certain, that they would not be for you?

O Brave New World!