tax exemptions

Divorce decree states if his exwife isn't working he can claim kids on his federal tax report.
For 10 years she hasn't worked, now she is.
We send her the required form in a self addressed stamped envelope, certified mail to inform us who we may or may not claim. She sends the form back unopened and stamped 'refused'. We recieved a letter from the IRS stating we could only claim 1 of the children for 1998, we now owe $2015.00 for last years return. The year before she sent back the form in May, a little late. Now she won't send anything and we have no idea who we can claim. We all live in the state of Washington. Is there any way we can legally force her to sign and return the necessary form? We are current on child support payments so there is really no reason for the exwife to withold this information. Any help would be greatly appreciated.