What to do if predatory journal won't withdraw paper after acceptance?
There are three issues here.  Have you signed a publication agreement, have you paid them, and has the paper actually been published yet?


If all three have happened, then you are probably stuck.  Retracting a published paper is a big deal, since it involves
changing the permanent scholarly record, and you can't do this just
because you regret the choice of journal.
If you have signed and paid, but the paper has not been published yet, then legally they have the right to publish it but you may
have a chance.  An ethical publisher would not publish a paper over
the last-minute objections of the authors.  (Of course the publisher
would be legitimately upset with the authors if there wasn't a
compelling reason for the withdrawal, but they wouldn't insist on
publishing anyway.)  Unfortunately, there's no reason to expect that
a predatory publisher will behave ethically, but it can't hurt to
ask.
If you have signed but not paid, then you have some leverage.  Legally they may be able to demand payment, but actually suing you
would be more trouble than it is worth.  They probably don't want to
publish the paper without being paid, so you may be able to negotiate
a withdrawal.  On the other hand, it's conceivable that they would
publish the paper in order to strengthen their legal/ethical case for
getting paid.
If you have not yet signed, then there's nothing they can do, since publishing the paper without your permission would be a
copyright violation.  You should be polite but firm, and they will
have to accept that you have withdrawn the submission.