HMG Senior Software Developer/Analyst interview questions

They had two rounds of interviews, one with them, and the other with the client. However, after the interview, they sent me an offer. I declined because I had other better offers. Instead, they kept on haranguing me for 2 days, texting me even on weekends. I told them I am not going to accept their offer, since I have made my mind to accept some other offer. As the last resort, they even threatened that I was being unprofessional by NOT accepting their offer and threated to put in some black list of whatever. And my crime? NOT accepting their offer, and refusing to negotiate further with them. Either they have misconception that accepting interview is equivalent to accepting offer, which is not, or they are just plain garbage. I suspect the later.