I saw a poster presentation given by an instructor who taught the entire course online except for the final. Before the final everyone was getting A's and B's. Her final consisted of the exact same questions she had used for her online quizzes that everyone aced. Well, the entire class failed the final.
This is why students have to pass the (proctored) final exam in my class to have a hope of passing the class as a whole.
And most of my assignments require personal anecdotes or are things that aren't likely to have shown up online.
It doesn't stop them, I'm sure, but it does slow them down a bit.