Spamming has also been used as a denial of service (“DoS”) tactic, particularly on Usenet. By overwhelming the readers of a newsgroup with an inordinate number of nonsense messages, legitimate messages and computing resources can be lost in the deluge. Since these messages are usually forged (that is, sent falsely under regular posters’ names) this tactic has come to be known as sporgery (from spam + forgery). This tactic has for instance been used by members of the Church of Scientology against the alt.religion.scientology newsgroup (see Scientology versus the Internet) and by spammers against news.admin.net-abuse.email, a forum for mail administrators to discuss spam problems. Applied to e-mail, this is termed mailbombing. The Usenet Meow Wars (around 1996) were DoS attacks on various newsgroups aimed at specific posters that disrupted the newsgroups where they were active. The DoS attacks launched by Hipcrime, which continue today, are more crafted as DoS attacks on entire newsgroups. The alt.sex newsgroups were rendered uninhabitable by commercial porn site spammers, partially for advertising purposes and partially to destroy a perceived free competitor. (This spawned the creation of the moderated, unspammable soc.sexuality newsgroups.)
Forged e-mail spam has been used as a tool of harassment. The spammer collects a list of addresses, then sends a spam to them signed with the name of the person he or she wishes to harass. Some recipients, angry they received spam and seeing an obvious “source”, will respond angrily or pursue revenge against the apparent spammer, the forgery victim. A widely known victim of this sort of harassment was Joe’s CyberPost, which has lent its name to the offense: it is known as a joe job. “Joe jobs” have been used against antispammers: in recent examples, Steve Linford of Spamhaus Project and Timothy Walton, a California attorney, have been targeted. Sometimes victims (such as ROKSO-listed spammers) are subscribed to lists that don’t practice verified opt-in, such as magazine subscriptions and e-mail newsletters, a practise known as subscriptionbombing.
Spammers have abused resources set up for the purposes of anonymous speech online, such as anonymous remailers. Many of these resources have been shut down, denying their services to legitimate users.
E-mail worms or viruses may be spammed to set up an initial pool of infected machines, which resend the virus to other machines in a spam-like manner. The infected machines can often be used as remote-controlled zombie computers, for more spamming or DDoS attacks. trojans are spammed to phish for bank account details, or to set up a pool of zombies without using a virus.
Source : wikipedia