It is unclear from your question whether the adjustment of status has been approved. If it has not been approved yet, send notarized letters of withdrawal to the USCIS, and to the EOIR. THere is no form, you just write a letter. If the adjustment of status has been approved, then my understanding is that you cannot withdraw it (too late). You can also make an Infopass appointment, and file the letter at the appointment.
As for the ex going after your parents, that is not how the I-864 works. Rather, it is a requirement that you reimburse the federal government for benefits which are paid to the person you are sponsoring. Your parents have no obligation (unless they submitted an I-864).
Should your ex get deported, you will no longer be held responsible under the affidavit of support.
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Not sure what you mean by your ex going after your elderly parents. The I-864 is a contract between you and the government that you will continue to support your spouse (even after divorce) for 10 years or when spouse becomes a US citizen (whichever occurs first) or else the feds can recover from you should your spouse becomes a public charge. But, certain state law regarding family issues, such as divorce and dividing of assets, interpret the existence of the I-864 form differently regarding spousal obligations. If your parents were joint sponsors they have the same obligations to the government.
If your spouse is deported then you and your parents if they were co-sponsors won't be obligated under the I-864.