Simple answer: Don't vote. Why provide legitimacy to someone who you think is not "good enough". Voting is not something you do by default. It is a choice.
Not voting is what people do by default. It's not an interesting choice.
I disagree - even in countries where voting is not popular, you usually get much more than 50% participation in the voting system. Less than 30% hardly qualifies as "default".
In the US, it's hovered right around 50% for a while, though it's climbed up between 55-60% for the past couple elections. More people don't vote than vote for any candidate. I would agree less than 30% isn't default, but 45% isn't less than 30.