MCM Midland Credit Management Inc Called my family back east to collect on credit cards that where paid off 5 years ago San Diego, California

This company for the last 5 years have sent me letter after letter,  calls that can be called harrising Daily. They called my sister in law back east to get the money from her.    They are trying to collect on 4 credit  card bills that I paid in full in 2008.  I have sent their letters back stating all the facts.  But the calls and letters still keep coming.  How do I get these jerks to stop,   I just want to slap them.....Do I need a lawyer??   I want money from them for all the stress they have caused this family......