How could he have prevented Owens from casting that vote? Keep in mind that a concession is just a tradition with absolutely no legal bearing.
Al Gore proved that when he conceded and then back off that concession.
Conklin said the state sent a letter to the House Clerk last week explaining that no winner had been determined in the 23rd District, and therefore the state had not certified the election. But the letter noted that Owens still led by about 3,000 votes, and that the special election was not contested -- two factors that legally allowed House Speaker Nancy Pelosi to swear in Owens on Friday.
http://www.syracuse.com/news/index.ssf/2009/11/its_not_over_recanvassing_shows_ny23_race.html