When you decline a LJBF a certain amount of tact has to be involved. Any overt 'in your face' response will prompt an equally overt confrontational response. The trick is to convey your non-acceptance of her offer in as covert a fashion as possible, but still courteous or at least "business-like."
The first obstacle men have to get over is that LJBFs are REJECTIONS. They are not genuine offers of some kind of enduring friendship. This goes back to what I've written about intergender "friendships", and a lot of AFCs get it into their heads that they're going to buck a trend and actually be 'best friends' with their LJBF girl. I've typed on this prior so I wont go back over that, but the natural inclination for most men when faced with a rejection - that most often comes after a very long period of "sniping" after her - is to opt for the path of least resistance and certainly the one which will make him and her the least uncomfortable. Women know this. This is precisely why a LJBF has been proven so effective for generations. It gives both parties an acceptable OUT, or on his part, an OUT that at least blunts the rejection.
The problem with all this is that the LJBFed guy is caught in the process without ever having understood that he's playing a predictable part in a social convention. So he sees the LJBF as an event rather than what it really is, a socially permissible, socially excusable, mechanism for female rejection. As a guy gets consistent LJBFs he begins to see the process, but all this comes after having had exclusively invested himself in the LJBF girl up until the point of the rejection. This is where the "frustrated" part of AFC comes from; his investment.
That's the first part; a man has to recognize the LJBF as what it is. This is part of the learning process because a guy has to also do some very important self analysis at this point. Most chumps will self-evaluate and try to find flaws in their sniping. "She might have accepted me if I had done X, Y & Z to prove I'm worthy." Rather, a guy ought to self-realize why he was in a potential LJBF situation in the first place. I'll tell you now, if you got a LJBF rejection, odds are you went about the process wrong. You sniped, you pined, you most certainly placed yourself into a position of qualifying yourself to her and thus handed her the frame from the outset. You most probably jumped past the uncomfortable sexual tension of attraction directly into the comfort of rapport and familiarity.
So then how do you go about responding to it? Some will advise the blunt "I have enough friends" line, but you'll deal with the social fallout of such an overt counter-rejection and most likely get the "you're an ass-hole response". Depending on how comfortable you are with that I'd say it's fair game, but don't expect her not to behave as though you're sulky and resentful. Most girl's recourse at that point would be to think all you were interested in was ƒucking her. I realize how sh!tty that seems, particularly when most guy's getting the LJBF are there after having tried for months to get to the point of pressing the issue of intimacy and applying all the effort and personal investments (not limited to just missing other better opportunities). How could she possibly come to the conclusion that all you wanted was to get in her pants? It's her only social recourse, despite all you did to "prove" yourself up to then.
There's couple of better ways however. One is allowing her to deliver the LJBF and let it roll off. You don't have to be a prick and say "thanks, but no thanks." You could simply let the rejection go and strategically withdraw - so long as you think you can do so. Cut off all contact and move on to spinning plates as you should have been anyway. This is simple pragmatism, if not a bit introverted, but the end result is the same - she gets the message that you're no longer wasting yourself on her as a cause.
The other way is a the assertive counter rejection. This is not an overt "I have enough friends" response, but rather a drawing of attention to the social contrivance she's using and explaining it to her in direct terms.
After her LJBF, you can say "I really wish I could be your friend, but I'd really thought we meant more to each other than that after so long, and honestly, I'm looking for more. Sorry, but I guess I was wrong about you."
I wouldn't use this verbatim as some kind of script to follow, but this approach effectively puts the onus of the rejection back on to her and makes her aware of the LJBF as a rejection. The idea is to defuse any "he just wanted to ƒuck me" ideas AND draw attention to it as a rejection. The problem with a LJBFs as a social convention for women is that it's gotten to a point where it's a default, autonomous response, and not a real rejection of intimacy. It's become such a useful tool that even women don't understand the latent function of it. When they're made aware of it, in a responsible way, recognizing the rejection aspect is unavoidable. Insomuch as it's a Man's responsibility to approach, initiate, be decisive, etc. with a woman, it should be incumbent on a woman to give him a straight rejection or acceptance of his approach. Unfortunately not all of us are mature enough at any given stage to do so, so we develop social contingencies to cope with uncomfortable circumstance.
All this said, even after delivering an assertive counter, you MUST stick to your choice. You can only walk away with your self-respect and her own respect for as far as you're willing to follow through with it. Cut off attention, focus on other things, take some time for yourself, analyze how you came to be in the LJBF position, etc. She WILL try to get you back as a friend, for her own ego preservation if nothing else. Do NOT allow this. It's not her punishment, it's not spite, it simple utility. The longer you entertain her the longer you will be paralyzed. You will be in limbo because you refuse to see her behaviors are her message, not her words. When extinguishing a behavior, in behavioral psychology, subjects universally attempt novel behaviors in order to reestablish a previous reward / reinforcer that prompted the prior behavior. People will do this too. The AFC will step up his efforts in new ways in order to prove his merit for intimacy, and women will be flirtatious and accommodating in ways they never thought necessary in order to reestablish prior attention levels they enjoyed before a takeaway. Be prepared for this.