Form 1446 is more about an applicant withdrawing, not a sponsor.
You said she's on a BVA... is that post the grant of another visa (maybe 300 Prospective Spouse?).
You are certain that there is no already-granted permanent visa in play?
If there's no permanent visa yet, then an outgoing sponsor can typically just write a letter.
Have a look at the online tool
here, and the subsequent instructions.
You're not cancelling her visa. You are simply stating that you are withdrawing sponsorhip.