Unfairly blacklisted

I once met one of the consultants of the gym company at westgate mall was asked to fill an application form and they told me will get free one month trial ,I did sign it but I did not give my banking details so that guy told me he will WhatsApp me for my banking details I said okay ,later that day he did WhatsApp me and I told him I can't take the membership since I dont feel like exercising any more he rep**** very angrily saying he will blacklist me I thought he was joking..until I started receiving calls from them asking for payments ,I told them what happened but no they didn't not listen to me ..nd still now i am still getting calls and messages from them.I won't pay for any membership since I haven't been there to the gym .please assist me coz I dont want to be unfairly blacklisted hence they have done that already they even asked me to pay +- R2500 to cancel the membership I won't waste my hard earned money .