I would class a spoiled ballot as a vote for none of the above as they are counted.A strange logic indeed.
If enough eligible voters who would normally vote Conservative don't vote /deliberately spoil their papers (because they feel let down by the party) and thus deprive the party of its majority you are saying those members of the electorate just don't matter? Quite a lot members of the electorate chose to spoil their papers (no vote) in the recent local elections - do those people registering a protest not matter
It is an eligible voter's democratic right to choose not to vote.
Staying in bed is not a vote and does not get counted.