Bullying me for more money.

I paid up and cancelled. Now they claim I owe them an early cancellation fee with interest accruing every month. And now I've 🐝 handed over to debt collectors. Appalling. They just do anything they want and bulky you out of ur money. According to my records I paid up in full and gave the correct cancellation procedure on an account that was way beyond 12 months old. They are ********s. How do I protect myself from such a big giant????? I hardly ever have money for food. I am falling apart from being harrassed.