If your spouse is in violation of an official divorce decree, can you claim your child on your income tax return?

If your ex-spouse is in violation of the divorce decree, you would have to take him back to  court to enforce that provision.  Although he may be in violation of one provision, the violation does not negate the rest of the decree.  If you are looking for a modification of the decree regarding who claims the child for tax purposes, the proper channel would be to file a Complaint for Modification explaining what circumstances have changed to warrant a change in the Decree.