Go to your extras in your MyO2 and click on the + symbol which will expand the options.
One of which is to turn off auto renew.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Currently using: Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)
As it's been a couple of days since you asked about your Extra, I hope you managed to cancel it ok in the end. It would be great to hear back from you.