Don't vandalize or smear dog shit or leave a burning bag of crap on her porch and ring the doorbell. You'd only be stooping to a lower level than hers if you did that sort of thing. And if you're smart, you won't put it in her mailbox, either. She'll know who it's from, obviously. And if she reports that to the police, you'll get in bigger trouble than you know. They consider that tampering with federal property, believe it or not, when you mess with mailboxes or "interfere" with mail delivery. You'll not only get a visit from the police but from the postal inspector, too, and you'll be very unhappy with the fines you'd have to pay.

Seriously, train the dog with some sort of deterrent. Or go tell the neighbor that you're going to begin moving every bit of poop her dog leaves back over into her yard or boxing it up and saving it for her.