I gave the green check to @Kate Gregory, because the last sentence of her answer went "click" and gave me the solution I sought.
My guess is that this person is not capable of imagining any down side to receiving help like this. It's a shame that you are stuck with having to explain that. (emphasis added.)
What went "click" is that I don't have to explain anything; the onus to explain is on her (and it is always her).
Thus, I should ask questions, in an incredulous tone of voice, until she goes away. Of course, first I politely decline help: Thanks, no need to bother, or whatever is appropriate for the specific situation. When the "helpful" person (PIA) persists:
ab2: What are you doing?
PIA: I'm just trying to help.
ab2: But why?
PIA: (Something asinine)
ab2: Why are you so persistent?
The point is, just keep asking questions. Do nothing except ask questions.
Thanks to everyone.
Note this is a community wiki answer, so anyone can chime in, but no one gets rep points.