According to the U.S. Citizenship and Immigration Services, a petitioner can terminate a family-based visa petition such as a fiance visa even if it is already approved before the alien fiance is admitted into the country. Since a visa petition is first handled by USCIS, and then by the State Department through the National Visa Center, and finally by the U.S. embassy, you should address your request for withdrawal of petition to the office that presently handles your case.
File a withdrawal of the fiance visa petition with USCIS if you believe your case has not yet left the Service Center that approved your petition. Write a formal letter indicating your desire to cancel your petition. Use your USCIS receipt number as a reference number to help identify your case. It may take a couple of months before you receive an official form confirming that your petition has been withdrawn.
Send the written statement of withdrawal to the National Visa Center if your petition has already been transferred there. The statement must be from and signed by either the petitioner, the applicant or the applicant's lawyer. Give the names and birth dates of both petitioner and applicant and your NVC case number. Explain that you want to withdraw the fiance visa petition and why. If a lawyer makes the statement of withdrawal, attach USCIS Form G-28: Notice of Entry of Appearance as Attorney or Accredited Representative to the letter.
Contact the U.S. embassy if your case has been forwarded there for an interview and adjudication. The petitioner must execute a signed, notarized statement that he or she wishes to withdraw the K1 visa petition. Mail the letter to the Immigrant Visa Unit. Fax the letter if you wish but the embassy must receive the original document before your petition is terminated. You can cancel the K1 visa this way anytime until the alien fiance(e) is admitted into the U.S. using the visa.