Threatening to blacklist for service I never agreed to

I received a call from a lady at business directory at the beginning of 2017. She offered to list my business on their directory at a fee. I made it clear to her that I was not in a position to take up such an offer. She insisted that she would send me a preview of what the ad would look like on their website once I agree to the offer. I told her that if she wants to send me a preview she can go ahead and do that but must only contact me at the end of the month to see whether I would be accepting the offer or not. The following week, she contacted me and I told her that i would not be accepting the offer.

Now months later, they keep sending me an invoice of R2500 and threatening to blacklist me if I do not pay this amount immediately. This is **************** as I very clearly declined their offer.

Please assist