Try repudiate (repudiation), meaning "to reject the validity or authority of; to refuse to recognize; to cast off or disown (a son, lover, etc)"
The third definition there hints at the meaning of refusing to acknowledge the continuing existence of something (a relationship, for example) that existed at some other time.
Synonyms include rescind, countermand and invalidate.
You could say something like "Your credentials are hereby repudiated."
Repudiation is a data security term, described here:
Repudiation: A repudiation threat involves carrying out a transaction in such a way that there is no proof after the fact of the principals involved in the transaction. In a Web application, this can mean impersonating an innocent user's credentials. You can help guard against repudiation by using stringent authentication. In addition, use the logging features of Windows to keep an audit trail of any activity on the server.
I would take the repudiation of credentials to mean that any future transactions would be considered unauthenticated and repudiable.