How can I turn off Siri's joke responses?
You can't.

Due to the design of Siri, you cannot change the way she behaves. This behavior is all run off of apple servers and is the same for every person. 

Siri is able to learn behaviors but that is limited to repeated behaviors like opening an app and the way you speak. 

So no,  because of the way that Siri was written, you cannot disable her "jokes". Sorry about that. 



EDIT:
Source: myself after listening and reading up on Apple's privacy info, and the Internet's many forum posts complaining about this topic.

Also, from more reading, I found out that people have varying amounts of times that this happens to them. Generally, people who don't mind it (like me) don't really notice it. People who are bothered by it appear to notice it more often, so it becomes more of a problem.

You can provide feedback about this to apple via apple.com/feedback