I think this article is about a criminal prosecution for defamation.
The word "querela" can mean (civil) lawsuit, but in the context of the article, I believe it means "complaint". Mignini and two police women brought the complaint of criminal defamation which would be pursued as a criminal case by a prosecutor. If it were not a criminal case, a prosecutor would not be asking that the case be "archived" (dismissed? or suspended?).
The Italian criminal law on defamation is contained in CP Articles 595 and 596-bis.
Google translation of Article 595:
Whoever, apart from the cases mentioned in the previous article, by communicating with more people, would damage the reputation of others, it shall be punished with imprisonment up to one year or a fine of up to EUR 1,032.
If the offense consists in the attribution of a given fact, the penalty is imprisonment up to two years, or a fine of up to EUR 2,065.
If the offense went with the press or by any other means of advertising, or in a public act, the punishment shall be imprisonment from six months to three years or a fine of not less than EUR 516.If the offense went to a political body, administrative or judicial, or to a representative or an established authority in college, the penalties are increased.
Source:
http://www.leggeonline.info/search.php?qr=diffamazione