Cellc contract canceled but I was still debited

Cellc is probably the most *********** and ********** telecommunications business ever... My contract expired last year June and when I called to give them a notice I in may I was told I was too early and I must call exactly 30days before, imanaged to get it canceled the following month but a few months after that I noticed that they where still debiting my account (*****ing my money).. Gave them a call and they promised they won't debit again but they still did

Went to the bank this January 2022 and blocked them from taking my money, now I'm getting calls from their attorneys saying I owe them 6k

When to the bank around December to block them