"As for me, I must esteem myself happy, to have been the first that rendered those fabulous nonsensical stories of knight-errantry, the object of the public aversion. They are already going down, and I do not doubt but they will drop and fall all together in good earnest, never to rise again. Adieu." (2.1.74.20)
Cervantes ends the tale of Don Quixote by returning one last time to his beef with the dude who wrote a knock-off sequel to Don Quixote, Part 1. At the end of this book, Cervantes makes it very clear that he has killed off Don Quixote and that the guy will never, ever return. In other words, he's telling any would-be fan fic enthusiasts to buzz off. He also tries to make it clear that he is the first and only author who ever wrote a legitimate story about Don Quixote.
No doubt about it, Cervantes is very possessive when it comes to his famous character, and he wants to make sure that no one ever writes a word about this dude again. It's fun to speculate on how he would have ended the book if he didn't have so much spite for the author who knocked him off. Then again, maybe there wouldn't be a Part 2 at all without that poor guy.