Authentication is seen as "entering the state of being authenticated." An opposite of authentication would essentially mean "entering the state of being unauthenticated". It is more common to refer to the specific process used and the specific context would determine the most appropriate variant. A few examples:
Your session is about to expire.
This certificate is no longer valid — it is has been invalidated.
You have been logged out.
This painting is no longer for sale because the authentication has been retracted; its authenticity is suspect and if it is a forgery it will be exposed.
Your credentials have been compromised. Please authenticate.
If you truly need to refer to a specific opposite than the two most logical words would be:
You can read more about the un- versus de- differences here on EL&U. It is worth noting that "deauthenticate" seems to be more common while "unauthenticated" is already a word and sets a precedence for using "un-".
In the end, if you will be using the word for any official documentation or publicly facing content you should check with your employer and manager for any official style guides. There is no one correct answer, here, and consistency is very important.
One last note is that the term "reauthenticate" has a similar problem but can be avoided by simply using the word "authenticate." Certain dictionaries do include an entry for "reauthenticate", however. I was not able to find a dictionary that included either "unauthenticate" or "deauthenticate."