"Unforgivable" would be a better way of saying it in English.
In English, "I won't forgive you" implies that there is a possibility of forgiveness which the speaker is excluding, whereas "unforgiveable" implies that there is no possibility of forgiveness.
Yurusanai = I won't forgive you
Yurusenai = I can't forgive you
Yurusarenai = You won't be forgiven
You'll hear all of these but the editor is the one who decides what English phrase to use. Besides, in English the shades of meaning between those 3 are not particularly important.