Jessica DeGroot, 28, and her brother Jeremy Jaynes, 30, both of North Carolina, were found guilty in November of violating the legislation, the first in the US to introduce custodial sentences for the worst offenders.
The pair had been accused of sending out over 100,000 spam messages in July and August 2003 through an AOL server based in Virginia.
At the time, the jury recommended that Jaynes, also known as Gaven Stubberfield, serve nine years in prison and that DeGroot be fined $7,500. A third defendant, Richard Rutkowski, was acquitted.
Trial Judge Thomas Horne this week dismissed DeGroot's conviction, on the grounds that there was no "rational basis" for it, according to reports. Judge Horne commented that the jurors may have been confused by the technical terms in Virginia's anti-spam legislation.
But the Judge upheld Jayne's conviction.