Defaulted me for No Reason

<p>Gym Company in Gandhi square(Previously Calton Gym) defaulted me for No reason and this is affecting my credit score now, I was applyying for credit and got rejected when I checked my report i found out that I am defaulted to this account, What happened, is that I had a gym account long ago about 2013 and I settled the account around may 2015. I even have a proof of payment and Paid up letter with me but they defaulted me without even calling to ask me, I contacted them and even sent them proof of payment they have not resolved my Bureau information, I am verry angry about this because it is making my life difficult, I contacted the Lady called Nikiwe on Accounts , she is very rude to me even after I sent her proof of payment and paid up letter. Please help me with this</p>