In reference to yerricde's suggestion as to how Sisyphus can end his punishment, Albert Camus would argue that Sisyphus would precisely refuse to do such a thing, because he does not wish to end his futile task.
The absurdity in Sisyphus lies in his continued defiance of the gods. Even in his punishment, he is defiant, because he refuses to accept this as a punishment; the gods want him to feel repentful for his actions through this futile labour, but Sisyphus does not repent. They want him to hope for an end to it, and each time the boulder rolls back down he feels despair as that bit of hope in him dies once more.
Sisyphus does not harbour hope; he knows that against the all-powerful immortals, he doesn't stand a chance. So maybe he can place the rock atop the hill without it falling back down, but what would that accomplish? To hope that the gods would then allow him respite would be silly, since Zeus isn't really a guy into that whole forgiveness thing. No, he knows that he will be punished for eternity, so ending this roll-rock-up-hill thing is just as meaningless as continuing through his labour.
That is what makes Sisyphus the Absurd Hero; he sees that everything is meaningless, yet in spite of that continues to roll his rock, loving his eternal struggle as his lack of remorse is an eternal revolt against the immortals; his lack of regret affirms the love of life he had and still has.