If they fail to vote in 2 consecutive elections AND fail to justify why they are not doing it. All you gotta do is pick up a form at the post office and fill it up, I doubt they even check it. But the thing is, most of us rather vote. When the whole Bush/Gore election madness went public on the news, people here in Brazil was not familiar with US election system, so most of us were shocked that people didn't have to vote (or even worse, didn't want to), that the majotiry of votes didn't elect a president, and that the whole voting system is outdated and confusing, using punch cards and manual handling of the votes, thus leading to mistakes and fraud...