Education is the best prevention.
Three resources that can be helpful in explaining to students what plagiarism is and how they can avoid it...
1: Plagiarism explained by Common Craft
2: Purdue OWL website
3: Plagiarism.org (produced by those who produce "Turn It in") and...
Five resources for detecting plagiarism...
1: How practitioners can use Google
2: University of Marylands Plagiarism Checker
4: Paper Rater - probably the one all learners should be encouraged to use to improve their "papers"
5: Plagiarism Checker.com (similar to 2 but produced by different organisations.