(I have been accused of having access to "secret girl lore" for saying what I am about to say.)
This advice is mainly for women trying to fend off overly enthusiastic suitors of the opposite gender. (Some of it can also be applied to breakups in circumstances under which respect is no longer called for.) (N.B. This advice does not address (and is certainly not sufficient for) creepy stalkers, abusive boyfriends, abusive ex-boyfriends, or other psychopaths. This is just about your normal, everyday, hopelessly-in-lust bonehead.) There can be exceptions, for extenuating circumstances, to everything I say below. Use your brain. Use the below advice only if rejection of a man does not look likely to pose a threat to your safety.
This is the key line. Bluntness is your best friend. Vagueness is also your best friend.
There are two very good reasons for this (one of which1 is too lengthy to include here). When you give a reason for not dating a guy, he will take it as encouragement. He will think, "Ah, so she'd be interested in me if it weren't for this one thing!" He'll see it as an obstacle to work around, not as a barrier to hope. This will make it harder for you to get rid of him.
Your hints are ineffective compared to the power of his wishful thinking. Guys are not mind readers at even the best of times. When they want something, they can find any reason to continue to hope for it. You may not think that you're "encouraging" him by being nice, making eye contact, or existing in the same hemisphere, but he does. You're not doing him any favors by being "tactful". Blunt honesty is far kinder, by letting him know where he stands, and keeping him from wasting any hope or emotional investment on a lost cause.
He'll only become more interested. That's right, it makes absolutely no sense. But he's not thinking with his brain; he's thinking with his dick. To a hetero male, lesbians are hot! Don't invoke those hormones.
If he asks you for a reason, circle back to your original statement.
"Why aren't you interested in me?"
"I'm just not. Sorry."
Telling him that you're "just not attracted" to him is acceptable. There doesn't have to be a rationale for attraction (it's better if there isn't one), and lack of attraction is usually seen as a nearly insurmountable barrier. But beware: A mightily thick-headed guy will still see this as merely a temporary obstacle.
1. The other reason (to not tell him *why* you're not interested in him) is that your stated reason is a lie. Oh, sure, you've convinced yourself that it's true, that since it applies to him and you're not interested in him, it's a valid reason for you to not be interested in him. (And this way, you won't have to actually tell him that he has the sex appeal of a garden slug.) But nine times out of ten, you're going to contradict that reason within two weeks. Maybe it'll be something you say without even thinking. Maybe you'll start dating someone else, to whom the same reason should equally apply. Either way, he's going to realize that you've (intentionally or not) lied to him, and it'll make him feel disrespected and trod upon. At the time that you say it, you'll think that you're sparing his feelings. But it'll end up with him feeling like dirt. So just don't do it. Don't give him the lame, easy excuse.
And seriously, if you're about to tell him, "I'm not ready for a relationship with anyone right now," take those words and shove them up your ass, then seal them there with Krazy Glue. No matter how true it feels, eventually it won't be. You are guaranteed to hurt him in the long run with those words.
