A letter from you to CIS to notify them that you and your fiance no longer intend to commit to the relationship is sufficient. As the US citizen petitioner, you can withdraw from the petition at any time. CIS will immediately revoke the K-1 visa and she can no longer use it to enter the country. The decision is not with prejudice and, if in future, you both agree to renew the relationship, you may file another Form I-129 F fiance petition for her.
Answered on Apr 11th, 2014 at 6:02 PM