Billing me for cancelled service

I really dont like doing this again, but I recieved a mail threatening me that in 48 hours I must pay an amount over R1000 or I will be given over, firstly you opologise for the bad service I recieved and now they just keep on billing me, I told you I am not paying for the service I recieved and yet you just keep on billing me every month and the amount just gets higher and higher, I mailed accounts asking a manager to phone me so we can sort this out but i never got an email response or call just got this threatening mail and that you dont do. I cancelled my service long ago and asked to be removed from web africa completely but yet this also didnt happen, I have no services with web africa but yet every month i get amounts higher and higher I must pay. So give me over and then youl see what a monster I can be which i dont want to coz I sent a nice mail asking if we can discuss this and i was ignored. Not my problem.