Is it permissible to publish a warning about a sorceress, or mobile phone viruses, or destructive links?

There is no harm in warning against evil and its perpetrator. Backbiting may be permissible or even obligatory for a valid religious purpose that cannot be achieved otherwise. This includes warning Muslims against evil and advising them, such as discrediting narrators and authors, or those who undertake to issue fatwas or teach while being unqualified, or mentioning the open transgressor of his wickedness or innovation for what he openly declared. However, accusing a specific person of sorcery without evidence is unlawful and falls under the evil suspicion forbidden by religious law.