constant harrasment on paid account

i have been having issues with Woolworths since 2012 unfortunatley they only know how to hire *********** staff. i was handed over to one of the useless debt collectors who also didnt know what they were doing. bottom line is i eventually paid my account and even requested for a letter from the debt collector confirming my account was paid in full and records to be updated this was in 2014 yet i still get them harrasing me yet when i want answers no one can answer me, i send emails as per their request yet they cant even respond till today i am still awaiting an answer. i have vowed to never set foot in a woolworths store EVER and i make it a point to bad mouth them to friends and family. This company seems to be very desperate to get money that they bother ex customers who have paid their accounts years back. Shame on you Woolworths hire people who actually know what they doing, otherwise stop harassing ex customers for your incompetency.