David Lennon, 19, was sentenced to two months' curfew, during which he must stay confined to his home for certain parts of the day, British media reported.
In early 2004, prosecutors said Lennon sent five million emails to Domestic & General Group, a U.K.-based warranty protection provider. The attack collapsed the server, causing the firm to lose $30,000, according to a BBC report.
Lennon was charged under the Computer Misuse Act of 1990, and authorities said this was the first time someone was successfully tried in the U.K. for such an offense, according to published reports.
Lennon had argued he was not violating any laws because the Domestic & General Group website invited email comments. A district judge agreed, but prosecutors appealed, saying the attack constituted unauthorized use.
The Royal Courts of Justice ordered the case for a retrial but none took place because Lennon pleaded guilty.
Click here to email reporter Dan Kaplan.