She told him no. She told him it was a sin. She told him she'd rather die than submit to him. It was July 5, 1902 when Maria Goretti, sewing alone, was approached and threatened by Alessandro Serenelli. When he failed, he first choked her, and then stabbed her 14 times. She died the next day. Maria was an 11-year-old girl.
Maria Goretti was chaste to the very end of her life. She was declared a martyr because she died for her faith in Christ--that following Christ meant not sinning, and that death is preferable to letting someone use her body.
Chastity isn't just "practicing abstinence". Let me back up for a second. One of the errors of modern thought (which isn't so modern, considering the number of virgin martyrs besides Maria Goretti venerated by the Chruch over the last two millennia) is that we should be free to use our bodies to satisfy our drives and desires. Such a terrible misuse of our bodies has led to a vast array of social ills throughout history (a discussion far too off this topic to get into right now); but society at large isn't the only victim.
Such a view of the human body also creates a great chasm within the self. The body is not supposed to be at the service of the will; it's at the service of God: "In the divine image he created them." Gn 1:27 It's the human will which is out of synch, and usurps the body for its own. I believe that on a very simple level, Maria Goretti understood this. Living a chaste life means understanding that our bodies are created for God's purpose. As such, we should have the highest respect for our own bodies.
"Young people, look at Maria Goretti, don’t be tempted by the tempting atmosphere of our permissive society, which declares, everything is possible. Look to Maria Goretti, love, live, defend your chastity." (Pope John Paul II, September 1, 1979) It's good advice for adults, too.
To really understand the human body and it's place in our faith, read The Theology of the Body by Pope John Paul II.