Ex accruing debt in former spouse's name

husband and his ex have been divorced for over 3 yrs. We recently discovered she had phone service in his name. We pushed her to cancel this and she did - but has not paid final bills. What can we do to get her to pay? In the past we paid off numerous bills already for credit cards, medical - all of which were her responsibility. She gets plenty of child support and works full-time so $ is not the issue - it is just vindictiveness.