Is it possible to “undo” the sacrament of confirmation.
No.
Looking back, I realize I may have been naïve or influenced by my parents to truly make a decision for myself.
Luckily, the Sacrament of Confirmation doesn’t depend on you “making an informed decision for yourself.”

Infants can (and often do) receive the Sacrament of Confirmation at the same time that they get baptized, so your informed consent (though desirable) was not actually required, as long as you weren’t actually kicking and screaming to get away at the time.
Now, I am confident I will never stray back to the church. I’d feel better if I was no longer associated with the church. I know this might be an odd request, but it is sincere. Marriages can be annulled; can confirmations?
You would have to prove that the Bishop was insane and unable to form the intent to give you the Sacrament of Confirmation, or that he had never been properly ordained as a Bishop, in order for such a declaration of nullity to take place.
Even then, it would be assumed that the Sacrament would work “ex operato” - that is, by God’s will, such that those who believed they were being Confirmed actually
were being Confirmed. It’s a very tricky thing, un-doing a Confirmation, and in all but the most obscure cases, I would consider it impossible. You have, in fact, received the Gifts of the Holy Spirit.
Instead of rejecting them, I would actually suggest that you try opening them up and putting them to use in your life. Even an atheist can make use of Knowledge, can he not?
