Direct opposites:
* Contradict: To state the opposite of something.
* Deny: To state that something is not true.
* Dispute: To argue against something.
Other words with similar meaning:
* Withdraw: To take back a statement.
* Recant: To formally take back a statement.
* Disavow: To deny any responsibility for something.
* Refute: To prove something false.
* Ignore: To pay no attention to something.
* Dismiss: To reject something as unimportant.
The best antonym will depend on the specific context in which "reiterate" is used.