I agree that there should be no need for a letter. Aside from the HR standpoint, it could also seem a little cold and insensitive. Just use the direct approach (since, the direct approach is obviously something she can identify with). Let her know, in no uncertain terms, that you're not interested.
Could it be that your hesitation to take action is due to having at least the slightest interest? After all, who doesn't want attention of this sort? Granted, the attention she's giving is more Glenn Close-esque. If you get home and a bunny is boiling on the stove, run- fast!
Seriously though, don't hesitate to let this woman know where you stand.
Now as I sit here and work, she calls! I'm actually talking to her and typing this!
HD, why have you been so quiet today? Why have you not paid me any attention?
IS THERE ANYTHING YOU WANNA SAY TO ME!!!!!!!!????????
(I reply YES...but this is not the appropriate place).
(Then I say I'm going to church to lay out an obituary that will take 90% of my night).
WELL, CAN I COME WITH YOU AND HELP?
(No, I don't need help with the layout, but thanks.)
SO, WHAT ARE YOUR THOUGHTS ABOUT YOU AND I?
BAM....my dumb, passive, "self" feels like if I say "it's not gonna work and you need to stop" her feelings will be hurt.
WHY DOES GOD GIVE PEOPLE A CONSCIENCE!? UGH!!!!!!!!!!!
And LL, NO I AM NOT INTERESTED (anymore). I thought I was and acted upon what I THOUGHT, but now I see I was wrong and it was a mistake.
Because I expressed an interest, I don't wanna be labeled as "the next guy who led somebody on"! You know!?