This is how you do it.
Thanks but no thanksThe deputy returning officer at your poll hands you your ballot and you hand it back. You can, if you wish, say “I decline” while doing this, but there is no requirement for any declaration, oral or written, under the Ontario Election Act. The DRO will write “declined” on the ballot, preserve it for the returning officer and note in the poll record that the elector — you — declined to vote.
via Ontario election 2014: How to vote for ‘none of the above’ — without spoiling your ballot | National Post.