Nobody should be forced, but if it's a right you care to keep, it might be worth exercising it once in a while.
So if I feel that all candidates are equally poor, I should go along and scribble on my ballot paper? Waste of time. The right to vote won't be removed simply because some people don't vote any more than the right to drive will be removed because some people don't have cars.
If it happened to any significant degree, they would have to care, because the media would hound and deride any party that didn't at least try to appear to care.
There's a huge difference between caring and saying they care, and an even bigger difference between caring and being able
and willing to do anything about it. The only way the parties would get more people to vote would be:
a) Make it illegal not to. Useless in practise, as that would just force people to do it - assuming they didn't want to break the law - and so remove the democratic part of it.
b) Change their policies. Perfect in theory, as if people felt strongly enough about one or more issues, they'd be far more likely to make the effort. I certainly would. Useless in practise, as every policy change made would lose as many party devotees as it would gain new voters.
Let's say that the parties all come out and say how terrible it is that so many people spoiled their ballots, will that affect anything? No.
Those people who voted for one or other party will make disapproving noises about how those wasted ballots could have prevented the 'other side' from getting into power;
Those people who spoiled their ballots can sit back and get a warm feeling from having made their own little protest;
Those people who didn't vote will still not vote;
The politicians will have gone through the motions, from a PR perspective;
and the whole thing will be repeated next time around.
I don't vote because I see no point in it, based on the choices I've been offered, not out of laziness. I voted once, the first time I was legally eligible, as I felt I had a choice between what the available candidates offered and based my decision on that. There is no way someone is going to convince me to vote purely for the sake of it, even less to spoil a ballot paper, as I haven't heard any convincing arguments as to why I should.