Agreed, the new "flat" web design pages where there aren't colorful buttons, and it uses subtle text links that you really have to hover over, is a lousy web interface. Give me cartoon colors and I'm happy.
From the "Payments and Transfers" main menu item, hover over that and select "Pay Citi Credit Cards" to go to that page.
In the row of underlined Text links, there is "Auto Pay" as a link.
This brings up your card or cards, and a button for "Edit Series".
In the page for "Edit AutoPay" at the very bottom of the page is "Remove AutoPay from this Card".
That should do it.
Welcome to the myFICO forums!
There is also a feature in Citi Auto Pay, where you can modify the autopay amount coming up. I have full-balance set up on my Citi MileUp, but wanted to take advantage of the 1.99% APR offer. So the last two payments I was able to just modify the payment to be less than Full Pay, more than Minimum Payment, and the next cycle proposed is Full Pay, unless modified again.