Rejecting someone you're no longer interested in romantically can be a challenge. I'm in my 30s and while I'm better equipped to turn someone down or reject them, it still is hard and can often hurt myself as much as the other party. Perhaps that is why many folks lie, avoid the issue, or ignore the person they're trying to let go. They genuinely do not want there to be any hard feelings.
However, this isn't always the case. Even when you're truthful, someone still feels hurt or slighted. That beautiful woman you had your eye on for some time has crushed your heart. Now you must recover and collect yourself.
Nevertheless, honesty is basically the best policy. Honesty with tact of course. There is no need to beat around the bush, tell a white lie, or make up a fake relationship. Just tell them you are flattered and appreciate their interest. However,you only see them in a friendly, platonic way.
Now people can be funny, and once again still feel crushed. Continue to be their friend and converse and interact like nothing ever happened. Should this person not get the hint and try to harrass you, then you may have to get harsh. You can ignore them then (but not for no good reason), ask them politely to leave you be, and keep your distance. The next step would be to contact the police for a restraining order, and some of your closest friends so you will not feel harrassed. You should not have to change your phone number, address, or daily life just because a rejection has gone ugly. Protect yourself, don't be a victim, and live your life. It's not always that serious anyways.
So yes ladies and gentlemen it is possible to gently and tactfully turn down a crush or admirer. I'm a very grown man now, I left the games on the playground. I feel those that haven't and cannot reject me in an adult manner have some maturing to do, plain and simple. I know that seems harsh, but its true. A true man or woman knows how to handle and properly socialize with other folks in different ways. It doesn't cost a thing to be nice and polite, but honest about it. Act the way you want to be treated. One has to behave like an adult to be respected as one.
Finally, treat them with respect with each passing day. Things may have gone completely south and you two no longer speak. However, you two should still be respectful and cordial when you see each other. It's all about respect, honesty, and proper communication, which are important elements in any relationship.
No one likes to be rejected. However, that's part of the game, the law of averages. Everyone will not like you. Just make sure you stay likable in the process of the rejection, whether you're on the giving or receiving end.