Bolton is still mulling a request from Arpaio's attorneys to throw out all rulings in the case.
Judge Bolton found him guilty for defying a federal judge's 2011 order to end profiling practices in Arizona, which targeted Latinos. But he continued the patrols for 17 months after it was issued and was reelected.
U.S. District Judge Susan Bolton dismissed Arpaio's conviction with prejudice, meaning the matter can not be tried again.
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"This prosecution is over", prosecutor John Keller said. The pardon had previously led the judge to cancel Arpaio's sentencing hearing. In the ruling, Bolton cited television interviews and news releases in which the sheriff made comments about keeping up the patrols, even though he knew they were no longer allowed.
The federal court order claimed Sheriff Arpaio was targeting Hispanic citizens for traffic stops on suspicion of being undocumented, leading to accusations of racial profiling.
Arpaio was convicted in July of willfully violating a 2011 injunction barring his officers from stopping and detaining Latino motorists exclusively on suspicion they were in the country illegally.
Chemerinsky had argued a contempt conviction is not an "offense against the United States" within the meaning of the Constitution's grant of pardon power in Article II. Arpaio, who was defeated previous year in the same election that sent Trump to the White House, is now talking about getting back into politics.