Blacklisting Final notice for Unused Service

I was approached by one of the Agents from the GYM Company in Maponya Mall. they had asked me to come join the gym, at that time yes I was interested so I signed the agreement and then a week later I called to cancel my membership because I had moved out of the area and it was a distance for me to travel to Gym and back. when I called, the lady who answered my call was not of good assistance, she insisted that I personally come to the gym and hand over a letter in order for me to get my account closed, she did not want to listen to me when I had repeatedly told her that I am out of Soweto and I cant afford to travel and bring a letter. Today I am shocked to receive a final notice for blacklisting, I am an honest person and I am a person who pays for the services that I use, I have NEVER set foot in that GYM till today so I honestly don't understand why I should pay R8070 for a service I never used. this company needs to understand customer issues and be able to deal with queries like my one, I think its very unfair to feel so threatened and such things will end up ruining my chances of getting a better job, car or home because I will have bad credit for something I did not use!!!