Perhaps I can offer a slightly different take. Maybe don't think of your AP as anyone different than any other woman out there (besides your spouse, by the way, just to be clear). And treat her how you would any woman approaching you and hitting on you (I'm inferring from your post that it is in-person, not by email/txt/etc.?).
With a healthy boundary with yourself, you can simply say, "thank you, that is nice, but I'm married and not interested". I think saying things like "Fuck off" are a bit elementary (initially, at least), and can serve as a disservice for you if you are a sensitive person. If, however, the woman keeps approaching you, contacting you, etc., after you made it very clear that you do not desire such contact, then you can escalate up to and including saying, "Fuck off". Because at that point, the woman is simply disrespecting you and your marriage (not to mention herself, but that's an entirely different story and thread!).
Remember, we are all human, and all sexual. It's perfectly normal to be hit on. Everyone likes it when someone says something flattering to them. It's what you choose to do with that action that matters. It's perfectly healthy to simply thank the person (especially if they have no idea you are married, let's say) and tell them that you are married and NOT interested. Do NOT say things like, "sorry, I can't, I'm married". That sounds as though you WOULD do something if it weren't for that darn stinkin' wife back at home!
Remember, how you say something can be MUCH more powerful and meaningful than WHAT you say.
Hope that helps!