there is nothing you can do to prevent the noncustodial parent from claiming the child on their return. if you file first that will only prevent them from e-filing. they can still file by mail. then the IRS will eventually request info to determine the proper taxpayer which should be you if they spent most nights of the year with you. however, if the divorce decree specified that the noncustodial parent gets to claim the child in certain years, consult your lawyer. while the iRS only says the custodial parent gets to claim unless form 8332 is provided to the noncustodial parent. the courts look to the legal documents.
