Honestly, although there may be a minority of women out there who deliberately lead men on, I think this is a situation which has someone become believed to be a pervasive problem, when I don't think it really is. It smacks of entitlement to women's attention and bodies and reinforces a kind of self-validating image of the man as irresistible - of course a polite smile means she's into you! - and the woman as devious and somehow at fault for the man's rejection.
In my experience, this type of situation is pretty gender neutral.
There are, of course, men who think that any woman who has a pulse and is in their presence is *obviously* hitting on him. And there are women who believe that every guy she meets is doing the same (I've met both types, and they are equally annoying. I've also heard other people talking about them behind their backs, and it wasn't pretty).
The crush situation is a bit different. Person A has fallen for Person B, hard. They are looking for any sort of evidence that their feelings are reciprocated, and will wildly over interpret anything even vaguely positive. If the two people have an ongoing friendship, there's ample chance to do that. Throw in a few Hollywood stereotypes (hang around being the best friend and she/he will eventually see that you're her/his soulmate, rather those other people actually dating
Some recipients of this sort of crush are totally oblivious (although their current partner frequently isn't), others see it, but aren't sure how to stop it (like the OP), and others, whether consciously or subconsciously, rather like the attention of a devoted follower.
From the point of the person with the crush, the only way to handle it is time and distance. So if you realize you're crushing on a person, and you realize that they aren't interested, you have to summon up the courage to back off and limit your time with them to group activities (no email chats, no one-on-one socializing). If you're the one who's being crushed on, and you've very clearly turned down any direct overture, and they're still not getting it, then it's in you're interest and theirs to do the same sort of backing off.
And it's definitely not kind to use that longing for your own benefit, even if you've told them no way directly. So no calling up the crushee for a shoulder to cry on when your BF has been insensitive, or asking the for help repairing your car, or a place to crash when you're in town, because that will just give them hope. Also no friends with benefits situation justified by "But I told them I'm not interested in a relationship
! If they didn't believe me that's not my fault!" For someone who is genuinely a platonic friend those things would be fine, but doing so with someone you know has an unrequited, hopeless crush on you is crossing over into leading them on.