Most people are getting hung up on laws. But elections are not about laws.
The purpose of an election is to confer a form of legitimacy on the winner of the election -- democratic legitimacy.
The laws and procedures around the election are setup to make the election seem legitimate. In some cases and systems, this also makes the election an actual reflection of people's votes.
Now, not voting is relatively common in wealthy western democracies. It is usually interpreted as apathy -- not caring who wins. A turnout of 40% or 60% or 70% of those eligible to vote may make the news, but as a minor point. Only drastic swings away from a previous vote total are really news worthy.
A massive campaign to not-vote, protesting the election itself as being illegitimate, can undermine the democratic legitimacy of the result of the election. This is done when a portion of society feels that the rules of the election have tipped the scales in favour of one result or another to an unfair degree.
Spoiling your vote sends a different message. Many systems record votes as being spoiled, and in some cases record refused ballots separately.
Spoiled ballots are usually interpreted as the voter making a mistake. In some cases, mass spoiled ballots are used as a sign that the vote counting wasn't legitimate, and triggers a recount.
Mass refusals of a ballot haven't happened as far as I know. But, in a system where it is or can be tracked separately, it would be a really clear indication that the ballot wasn't considered legitimate by the people getting ballots.
An example of where that might be used is when the people writing the ballots have eliminated an option that would win. Not showing up would be one option, but showing up and declaring the ballot illegitimate by declining it en-mass might make the result of the election seem extremely illegitimate.
The down side is that the people setting a ballot are usually those who count the ballots, and often have control over the media messaging around the vote. Getting the message out "show up, decline your ballot" and having that accurately counted to the point where the election is clearly illegitimate would be quite a challenge.
TL;DR - voting is a form of power and expression. Not voting and spoiling and declining and voting are all different expressions, and can be interpreted differently. Under most voting systems, only those who vote are counted for the "winner", but the purpose of winning an election is to claim democratic legitimacy.
A country with 30% turnout can have the winners of the elections claim democratic legitimacy.
A country with 60% turnout, where half of the ballots are refused or marked "none of the above", is going to have a democratic legitimacy deficit, even if it picks the same winners.