You cannot stop her. But you can print, sign and mail your own tax return that claims the child, and the IRS will send letters to both of you to sort it out. Make sure you have good records from day cares, schools, doctors, dentists, church, etc. to prove that you are the one who had the child in your care and that he lived with you for more than half the year.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
