How can I stop them for calling me ?

Multiple people Academy Collection Service keep calling my number and leave messages with a call-back number in my voice mail for about a month. I called back once when I received their first call and a lady from ACS received my call. She (and also all messages in my voice mail) even couldn't get my name right (only know half of my first name), but she wanted me to provide my social security number ! I certainly can't give to them. Then, the lady said bye-bye. They still keep calling my number and continue to leave message for almost a month now.

I never have any debt in my life and have just checked all my credit reports and they are no problems! How can I stop them for calling me ? If they really have something on me(which I don't know and must be a fraud), why they just send a mail with details? :evil: :(