How do I prevent a offset for child support debit?

[Critter-3]

If the IRS has an offset on the books then any and all refunds will be grabbed to satisfy this debt. They can even take state refunds.    Now are you married and this debt is not yours but is the other spouse ?   If so then if you file a joint return you can also include the form 8379  (Injured Spouse)  if you qualify. 

Form 8379 lets you (the "injured spouse") get back your portion of a refund from a jointly-filed tax return if it's seized or offset to pay your spouse's debt.

You must file jointly to use this form. Filing an 8379 will delay your federal refund by up to 14 weeks.

To file this form in TurboTax:

1. Open or continue your return.
2. Go to Other Tax Situations
3. Scroll down and select Other Tax Forms 
1. Report an injured or innocent spouse claim ... say Yes to Do you want to claim innocent or injured spouse relief? and carefully follow the onscreen instructions.
2. Please be aware that when you get to the Tell Us More About Your Situation screen, in order to meet all qualifications to file the form, the box I am not required to pay the past due amount must apply to you. If so, check the box.
3. Note: Pay close attention to the screens, as some of them apply to innocent spouse relief, which is different. Leave those questions blank if you only want to claim injured spouse relief.

[Opus 17]

If the debt was incurred by you, you have to pay it one way or the other.

If the debt was incurred by your spouse after you were married, and while you were filing joint tax returns, you are jointly liable for the debt and you have to pay it one way or the other.

If the debt was incurred by your spouse before you were married, you can file form 8379 Injured Spouse Allocation, to request the IRS protect your share of a joint refund.  Your spouse's share will still be taken for the debt.  You can file form 8379 with your tax return, or you can file it by itself once your tax return is processed and you discover your refund was taken. 

https://www.irs.gov/forms-pubs/about-form-8379