U.S. Immigration Law -- Can the petitioner withdraw(or nullify) an
already-approven immigration case?

I am a naturalized U.S. citizen. A few years ago I filed an
immigration application for my mom in China. The case was approved in
2004 and my mom also got her visa in early 2006. She's planning to fly
to the U.S. in June 2006.
Recently she has been behaving irrationally and I am very worried that
her coming to the U.S. will bring severe trouble to my family, friends
and maybe even the public.
I wonder that if it's possible for me to withdraw the case under such
situation so that she cannot come to the U.S. freely. That is, even if
she comes here in June, her green card will not be issued and she has
to go back to China after a few months for example.
If this is doable, what is the usual procedure?

This is a free comment, not an "answer" to your question.

You might consider increasing the price of the question considerably
to urge a Researcher (one with a blue name) to look into this,
although that will not guarantee that you will get a positive answer.
I have tried to make a quick search, but only found information
relating to potential terrorists.  I expect that your best bet is to
call the office where you made the application and explain the matter.