So I called and argued for 20 minutes about how I had cancelled that plan a month ago, and they continued to say "No you didn't."
Well I did. I had a whopping .56 megabytes used on the day BEFORE I cancelled my plan, and after that, there was no data activity. None.
I finally broke down and pulled out my big guns. "Well, then I'm just going to have to consider cancelling my plan", which they followed with "Okay, let me get you to an account specialist."
He immediately realized "Oh shit he's not kidding", and fixed my bill, removed the $20, and apologized profusely for the misunderstanding.
I hated pulling the "I'm gonna cancel" card, but in this situation, it was warranted.