BOSTON – A man who had been convicted of drunken driving six times had his seventh conviction thrown out Tuesday by the Massachusetts Appeals Court, based on what the court called an illegal stop and seizure by an off-duty police officer.

The court set aside the conviction of Joseph Limone, ruling that evidence found after the stop should have been kept out of his trial.

According to a summary included in the court’s ruling, Limone was driving in Woburn on Aug. 4, 2006, when his car struck a vehicle driven by Robert Kelleher, a Somerville police officer who was returning to his home in Woburn after performing a paid detail.

Kelleher believed Limone was under the influence of alcohol, took the keys from his ignition and called Woburn police.

Officer David Simonds responded and asked Limone for his license and registration. When Simonds asked Limone to step out of the car, Simonds said he saw a nip of whiskey in Limone’s lap.

Limone failed field sobriety tests, and after being arrested and taken to the police station, he recorded a breathalyzer reading of .12, above the legal limit of 0.08.

A later check of his probation record showed six previous convictions of operating under the influence.

In his appeal, Limone argued that evidence used by the police to prove his guilt was obtained as a result of an unlawful seizure.

The Appeal Courts agreed and said the evidence should have been excluded.

“What the court is concerned about is if we permit people who are acting as private citizens — as this off-duty officer was … then that opens the door to vigilantism,” said Robert D’Auria, Limone’s lawyer.