Is warning people about a person who commits evil a form of slander or       backbiting?
Question

                If a person is known for his evil deeds and he openly commits major sins such as drinking alcohol etc. and we advise others to keep away from him and to avoid dealing with him, is this permissible?
Praise be to Allah.Warning
      others about a person who does evil and commits major sins is obligatory
      (waajib). If a Muslim fears that his brothers may be harmed by those
      people’s actions, he must advise them and tell them about these
      evildoers, so that no one will fall into their trap. It is also obligatory
      to advise the evildoers and those who commit major sins, so that perhaps
      Allaah may guide them. The Messenger of Allaah 
 (peace and blessings
      of Allaah be upon him) said: “Religion is sincerity” They said, “To
      whom, O Messenger of Allaah?” He said, “To Allaah and to His Book, to
      His Messenger, and to the leaders of the Muslims and their common folk.” 
(Narrated
      by Muslim from the hadeeth of Tameem ibn Aws al-Daari (may Allaah be
      pleased with him).