You are in a catch 22. She is in violation of the court order, if she does not allow you to claim the child, however, if she is the custodial parent, you cannot claim the child without an 8332 release. I suggest you work out an agreement for her to sign the form and you to apply all the money to your arrears.

http://www.irs.gov/pub/irs-pdf/f8332.pdf