Nehaji,
Your question is not clear. What do you mean of harassment? Is he harassing you outside the court for getting ex-parte decree or HAS HE FILED ANY APPLICATION IN THE COURT FOR SETTING ASIDE OF THE EX-PARTE DECREE AND YOU ARE CALLING IT HARASSMENT?
If he is harassing outside the court, he is no more your husband and hence you have to file a police complaint against him. You have also got the option of invoking the provisions of Domestic Violence Act even against the ex-husband for harassing you. You will be given protection.
If he has filed application for setting aside of ex-parte divorce decree in the competent court and you call it harassment, then you are wrong. He has a legal right to move such an application as per law. What you have to do is to engage a competent advocate to reply to this application and argue forcefully, that there was no fraud committed by you to get ex-parte decree.
Wish you best of luck.