You might intend it as that, but that isn't what is being heard.
Spoiled ballots are an after thought, a minor inconvenience of a 100 or so ballots that can't be counted for anyone of literally dozens of reasons.
Using spoiled ballots as a protest, at least on an individual level, is utterly pointless because it will be drowned out by the noise of every other reason a ballot might be rejected during counting.
If you could get a lot and I'm talking at least 10% of a constituency to spoil ballots then someone might pay attention.
I'd also fundamentally disagree that vote spoiling is an expression of contempt for the system because you are fundamentally, on some level engaging with the system. The ultimate show of contempt for a participatory system is, of course, to not participate.
Are you making an assumption I support the major parties? You genuinely couldn't be more wrong.