That can definitely be a tough question... at least at first. Sometimes, if you wait long enough, their persistence becomes so annoying that it's very easy to drop all concern about appearing jerk-ish, and just make it VERY clear.
Of course, that's obviously not the best-case scenario. So, here are some strategies.
- Say no. Again. Of course, you should do so very politely. "I'm really sorry, but I'm not interested." And if they view that as leaving some sign of hope, "...and I don't see myself becoming interested anytime soon/ever." That may be pushing into jerk territory, but you've gotta do what you've gotta do. You don't want this phenomenon...
- Prove it. Having someone on your arm is probably a good indication that you're choosing to ignore said admirer's obvious interest. And if there isn't a readily available girl/boyfriend for you, making it clear that you're interested in people who aren't this person-who-doesn't-know-what-no-means is another alternative (a.k.a. saying something about someone you like in earshot of the person who likes you, or just straight out saying "I like someone else. Named ____," to them. But naming names is obviously optional.)
- Ignore it and hope it goes away. My personal favorite, and probably the least mature of the three options, is to make your feelings known in a more physical and psychological way; shun. If they're always trying to spend time with you? Always have somewhere else you need to be. If they're always trying to chat with you when you're online? Well, that's what Facebook-chat ignore lists are for. Constantly texting you? Respond with those irritating, conversation-ending one word replies. If you're in a position where you can't readily escape, but instead have to endure they're attempts at conversation? One-word replies came from somewhere, and cell phones didn't always exist. You don't have to cop an attitude, but by seeming less than entertained by the whole deal (in a very polite way), you can get the impression across. Now I'm sure none of this seems very nice, so I would suggest it as a last resort; this is for when you begin to get very, very fed up. Like when the person is becoming borderline obsessive (or at least it feels that way).
Now, these aren't the only three ways to deal with this kind of situation; they're just the three that I utilize the most. If none of them seem very plausible to you, then I would suggest sitting down and having a brainstorm session with yourself, because you're the only one who knows how you'll be most comfortable when dealing with any given issue.
Additional Notes:
In the unfortunate circumstance that this person who won't listen is a friend, I would suggest doing whatever you choose to do as quickly and painlessly as possible... drawing it out could really wreck your relationship.
In the equally unfortunate circumstance that this person is someone a friend is interested in, I would worry less about painlessness than about CLARITY. "No, I am not interested, and no, I will not be, and no, I do not want you to profess your love for me in public; that will not change my mind, it will just make me mad." At this point it doesn't hurt to suggest they be open to other people (*cough* such as the friend), but you don't want to be too blunt about that. Clarity with your friend is very important, too. It may not be your fault that this person likes you, but it WILL be your fault if your friend thinks your inaction is leading them on, or is an indication that YOU like them in return.
I wish you luck!
