This is very difficult to answer, if you don't tell them, you'll hurt them but at least they will be able to move on. If you ignore them, it won't hurt them that much, bit they won't be able to move on before knowing what you feel.
I think you should just ignore them.. if you go and say anything to them at all.. it would then seem to me that you just want to keep something going with this person and cause more heart ache to you.. because the person you said it to.. my thinking is.. say to him or her self.. I got under their skin now I think I will keep it this up and that is most likely what they hoped for anyways from the start.. but if you just ignore and do not even worry about that person at all.. there is no backlash.. period
I tell them. And I make sure not to leave them a way to misunderstand or think they can make me like them just by trying. It's the fastest way to stop them from bothering me, if I ignore them but can't distance myself from them (Example: A coworker) they'll still try to be friendly or keep up appearances. Flat out rejection is best - quick, easy and hassle free.
Eh well lets see... If I don't like them at all I probably will flat out not know they exist. If I like them, but there are things about them that trouble me then I usually express my dissatisfaction bluntly... Though I should probably be more sensitive, I like to keep things simple and honest with people I care about.