Make sure you're certain you really want them to stop liking you. If you're not sure, you'll put yourself and the other person through unnecessary pain and send them mixed signals if you change your mind later. Think about why you want to end the relationship and write down your reasons, or talk it over with trusted friends. Keep clear boundaries with the other person so you don't confuse them. This will usually work well enough so that you don't have to have an uncomfortable conversation, but sometimes even if you try to maintain boundaries they'll still try to hit on you or make inappropriate comments. Some good boundaries to try out before talking to them are: Avoiding touching them or flirting with them. Unfollowing them on social media. Avoiding spending one-on-one time with them. Staying busy with other people and friends. Saying "no" to any plans they try to make with you. Avoiding sitting next to them at work or in class. It might be tempting to try to get them to stop liking you by fighting with them, acting out, or turning them off with bad habits. But that could take a while and will be more painful in the long-run for both you and the person you're dating. They'll probably know you're trying to push them away, so stop playing games and go ahead and talk to them directly! If they don't respect your boundaries, or can't seem to take a hint, you'll need to meet them in-person to talk. Be clear and be firm that you're not interested. Although it may be uncomfortable to tell them you're not interested, especially in-person, they'll be more likely to believe you than if you just send a text message. If you really haven't known the other person for long and haven't spent much time in-person together, then giving them a call or sending an email would be appropriate. You don't have to be brutally honest, but they do need to leave the conversation knowing you don't like them in that way. You don't have to list all their faults, but make sure you're prepared with specific and honest reasons about why you don't like them. Avoid vague statements like "it's not you, it's me." Try to keep the conversation low-key. If you've only known them for a few weeks, you can say, "I think you're a good person, but you and I have completely different interests, friend groups, and lifestyles. I'd rather just go our separate ways." If you've known the person for longer, organize your thoughts beforehand. Be clear in your own mind about what exactly led you to this decision, whether it's because they're a really good friend, because you're with someone else, or because they're a co-worker. Whatever your reasons, you can keep saying to them, "This isn't working for me." This will keep your conversation with the other person on track. Even though you want them to stop liking you, you can still be nice to them by understanding their feelings and perspectives. The other person may try to persuade you that you're wrong or that they can change. Acknowledge how they're feeling. Just don't let their emotions sway you to change your mind. If you've tried talking to them and they're not listening, still trying to win you over, or still disrespecting your boundaries, you'll need to talk with them again. Make it clear you don't share their feelings and need their behavior to stop. Repeat and reinforce what you've already told them before and keep maintaining your boundaries with them. Try to write them an email instead of talking to them in-person, this way you won't have to be alone with them. Try to address your concerns as they come up. For example, if you notice the other person looking at you or saying inappropriate things, you can say, "I'm not comfortable with that," "Please stop, I'm not interested," or "Please don't say that to me." If all of this fails and the other person continues to give you unwanted attention, get help from others. Let your parents, friends, teachers, and/or counselors know what's been happening. Make sure you keep your distance from the other person, but if you have to be around them make sure you're not alone with them. Keep a record of any texts or emails they send to you, especially if they're inappropriate. Make it clear to your supervisor, parent, or other authority figure that you've done everything possible to discourage the other person's attention and behavior.

Summary:
Be confident in your decision. Set appropriate boundaries. Don't force their hand. Have a talk in-person. Be specific, honest, and clear. Be nice. Reinforce your position. Get help.