Yes, there's a "Blank" option when voting, and there's also a "null" vote, which happens if you vote for a number that's not in the running (each candidate has a number. For example, in this running Lula is 13 and Bolsonaro is 22). Only valid votes are counted, so whoever gets anything over 50% of the valid votes win.
I'm seeing 3 million Nulo votes, which is way more than the margin of victory. Interesting.
It's always an option, I believe. You must vote, but you're not forced to vote for any candidate.
It's important to say that if you fail to vote, the fine is R$3.50 or around US$0.66.