BLACKLISTED!!

My grandmother ( Ellen Modise ) purchased a phone at Mr price using an account, a few months after purchasing the cellphone, the cellphone began giving her problems. We went to the store and reported the matter, they took the phone for repair, and brought it back to us, but it still gave us the same problems. we went back to the store they took the phone again, for "repair" it took maybe 2 months for them to bring the phone back, when they brought it back it still gave us the same problems and this time around it had gmail accounts and sim cards we didn't know. mind you during this course of the time, my granny stopped paying for the cellphone cause she saw this pointless, paying for a cellphone that is not with her nor working. we reported the matter the 3rd time, they told us stories, we kept the phone this time around. and she started purchasing the cellphone avoiding being blacklisted, she finished the debt. a couple of months she went to Mrprice to purchase something, they told her they closed the account cause she owes the store. she asked herself how cause she had an receipt stating she owes nothing. she went again last week to purchase something they told her the same story. she went to a different store to open an account, she find out that she's blacklisted. Can you kindly help with the issue.