Unfair labour practice and defamation of character

My sister resigned from Clicks Lichtenburg Mall because she got a better offer elsewhere and she has recently found out that she has been blacklisted on the Clicks Recruitment system. When she tried to establish why she has been blacklisted, she has been told by HR that they are still collecting information on why she has been blacklisted. since when resigning is a crime? this is unfair labour practice, defamation of character and her right to freedom of association has been trampled upon by the Clicks Group.