How do I stop the auto refill on a Go Phone Account?

I purchased an AT&T Go Phone for my 88 year old mother as she has been in an Assisted Living Facility where her Verizon cell phone wouldn't work. She is returning back home in 2 weeks and I need to cancel the auto-refill for $25. a month that we had set up for the phone. How do I do this? I've called several numbers, and can't get in contact with a REAL human being to ask questions and, so far, all I can locate online is how to set up a refill, NOT how to cancel one!!!  Thank you! Colleen