What exactly, and I ask this sincerely, would a spoilt ballot do? The votes will be counted, whichever party introduced as your government and the spoilt ballots will get a quick mention and perhaps an interview or two by some mouthpiece about 'hoping we can convince people next time with our policies'. By Monday it will not be mentioned again.
By spoiling your ballot you are effectively saying "I'm a potential voter who cannot accept any of the parties as they are" but by not voting you are saying "I don't matter"
Spoiled ballots give the parties incentive to try harder as they know you are a potential vote. Where's their incentive to try and appease you if you can't be relied on to make the effort to go to the Polls?