Confess means to admit that one has committed a crime or done something wrong; to acknowledge something.
Disavow means to deny any responsibility or support for something; to disclaim or reject. Since it is the opposite of admitting/acknowledging, it is the most appropriate antonym.
Why the others are wrong: * Witness (to see an event) is unrelated.
Scare (to frighten) is unrelated.
Preface (an introduction to a book or speech) is unrelated.