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.

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 a 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.