Woolworths bad service

I had a Woolworths Credit Card. Due to my knowledge and proof my account was fully paid up. Suddely Woolworths sold my account without my knowledge to Nimble Group starting with the original balance. Who gave Woolworth the legal right to sell my account and then it was paid up. Now I struggle to get rid of Nimble Group and this is effecting my credit score. Please be careful of Woolworths if you want a Credit Card. Stay away!