>Why don't just send them a letter and block their debits on your account?

Bell Canada and other big Canadian corporations have direct access to credit report records. If not paid, they will spoil your credit report. A credit report is essential in Canada for things such as credit cards, mortgage, car loan, and sometimes even for renting an apartment or home. With a poor credit report score all this would become more expensive or even unavailable.

Customer service provided by large corporations in Canada is sometimes absurdly poor, but you can't do much about it because there are simply no reasonable alternatives.