So, when do you just say no?

I had a patron the other night that wanted "5 books on segregation, congregation and disintegration and if you can just get those for me". After some questioning I figured out that apparently she wanted books on segregation in prison (there aren't any) then she told me she just wanted the numbers for books on segregation. Ten minutes later she came back and said, "Jesus is teaching me a lesson in patience today"...mutter mutter...and then something about how I had sent her to the wrong place. She then tells me she had asked me for books on prison and to give her the call numbers for books about prison. I said it is a broad topic and can she give me some more information so I could narrow it down and find the "right" books. She said it didn't matter, she can write a paper using any books about any aspect of prison and just give her the numbers.

My question is, if she came back again, what should I have done? She was highly agitated and impatient and was perhaps having a slightly different experience of reality than most of us. Do I put up with her abuse? (She was extremely rude to me) This is obviously not the first time someone has been rude to me while working this desk, but is there a point where I put my foot down and just say I can't help her? I gently suggested she try the OPAC and you'd have thought she suggested I sell her child to travelling gypsies. There was no way to make her happy, so how could I politely say no?

Any thoughts?