Nexon Enterprises — they are sending notice

I got call from them a months ago to do captcha work for their client. I got link from them through i registered and they sended me a username and password from that logged in to that website then i didn't understand what to do then i contacted them regarding this issue but they didn't provide me any response after few days they sended me notice in that they are mentioned that if am not obeyed to pay the penalty amount of 5000 means they are going to take legal action against me. Just now i received the notice from them. It's totally wrong way they are going