Glocell blacklisting

Actually no star last year I may 2018 the call centre told me my contract is finished I must just send a mail to confirm that I would not be going on with the contracr.I did that.today I find out I am blacklisted for R227 not a call or email or letter saying I still owe that amount.The simply just blacklist me .Who protects us customers from things like this or can companies do just what they want to.I am millions strong but get blacklisted for R227 what a joke if they called me I would have paid it immediatly.I am very unhapoy