Can someone help me close my account once and for all?

Good morning-

I'm looking for a way to close my AT&T account once and for all.  I switched to Project Fi in August, 2017, but despite my repeated efforts to close my AT&T accounts, the company has been unresponsive and now is trying to send me to a collections agency for an account I no longer have or want.

I called AT&T twice in August to cancel my account, and then discovered in September that AT&T was still billing me for a tablet, which I thought was also cancelled when I told the customer service agent to "close my account".  Needless to say, I paid for the month of August/Sept anyway thinking the misunderstanding was perhaps my fault and asked a rep to please close my ENTIRE account.  In October I received another bill.  I called AT&T a fourth time, and the customer service rep told me that the account was cancelled, but that the last payment for Sept/Oct was made "retroactively."  He assured me he would cancel the request for payment.  I also called my credit card company and told it to block AT&T from continuing to charge me at that point, since AT&T had "mistakenly" charged me two consecutive months.  Well, November has rolled around and AT&T has attempted to charge my account again.  Since the credit card is blocked, the AT&T has now sent me an email, threatening to send me to a "collection agency." 

The reason I am in this forum is because I can't figure out how to actually close my account.  Every time I call AT&T, I am put on hold for 15-30 minutes, then I speak with a customer service rep who assures me everything is solved - but then I get another bill at the beginning of the month. Is there a secret code word I need to say for AT&T to let me finally close my account?  I have to say, the collections agency isn't a good way to cross-sell or up-sell me on new AT&T products.

Thanks,

Current Project Fi and hopefully soon-to-be former AT&T customer