Infuriating, never ending cancellation process.

I was so happy with this provider for the first year and a bit, great quality internet and reliable. Service was also good... WHILE I was a customer...... Then they decided to change their pricing without my consent so I cancelled the line and paid the last month. They have since been harassing me non-stop and threatening to blacklist me even though I still paid the last month for service once I had cancelled the line. What's even more irritating is that I have multiple people contacting me over phone, email and sms. One to ask politely why I cancelled, another to confirm I cancelled, another to say I haven't cancelled, and then another saying I owe money and they keep invoicing me! I'm at my wits end here! Won't be going back to them any time soon.