You cannot stop someone from completing and filing a fraudulent or incorrect tax return.
All you can do is correctly complete and file your tax return. If the individual files a return and it is accepted by the IRS before you e-file your tax return, then your return will be rejected for e-filing since a return with the Social Security number of the dependent has already been entered into the IRS systems. You would have to print and mail your tax return to the IRS and include the dependent on your tax return. If you are owed a tax refund the IRS will send you the refund.
Within a year the IRS will notify each taxpayer requesting that they need to amend their tax returns if they incorrectly claimed the personal exemption of the dependent. If neither taxpayer amends their returns then the IRS will again notify each party that the IRS will require proof that the taxpayer was eligible to claim the dependent. You should respond immediately with the proof that is requested by the IRS. The party that was in the wrong will have to repay any tax refund and also pay penalties and interest on the taxes owed.
If you know who the other party is claiming the dependent incorrectly, you may want to let them know the consequences for doing so.
If this is a matter of identity theft then you can complete an IRS Form 14039 Identity Theft Affidavit, so in the future an Identity Protection PIN would be required with the tax return that is claiming the dependent's personal exemption. Go to this IRS website for Form 14039 - https://www.irs.gov/pub/irs-pdf/f14039.pdf
