Disaffirm

Repudiate; revoke consent; refuse to support former acts or agreements.

Disaffirm is commonly applied in situations where an individual has made an agreement and opts to cancel it, which he or she may do by right—such as a minor who disaffirms a contract.

A disaffirmance is a denial or nullification of the existence of something, as opposed to a revocation, which is the breaking of an existing agreement.