After all, a larger vocabulary is a good thing. Also, I use this to teach word stress, and the sounds of English.
When I have time, I do this as a class assignment, like this:
1) Transcribe the words from English into International Phonetic Alphabet (to show pronunciation) and mark which syllables are stressed.
2) Put that transcription on the board**.
3) Look up the meaning of the word.
4) Use the word in a sentence. (A sentence they come up with. I can usually tell when they just use the example sentences from the dictionary.)
As you can imagine, students sometimes resist this.
A few years ago, we were doing this assignment, and this particular student had the word "correlation". After a few minutes, I noticed that, while her classmates were busy doing the assignment -- a few were already at the board, in fact-- but she was just staring blankly into the middle distance.
Me: (Student's name), what are you doing?
Me: You have an assignment. Get to it.
Student: Oh, I don't need to know this word.
Me: (perturbed) Really?
Student: I already know everything I need to know.
Me: Then why are you here?
Me: If you already know everything you need to know, you shouldn't need any more education, so you shouldn't be here at all.
Student: (blank stare)
Me: The fact that you are sitting in a class means that you admit you need to learn things.
Student (a little defiant): I'll never need to know this word! If I needed to know this word, Jesus would have already made me know it.
Me (annoyed and a little angry): STOP! Just, stop right there.
Me: Do not use Jesus as your excuse.
Student: Well, Jesus...
Me: No, no, no... When Jesus was hanging on the cross, he was not thinking, "You know, I'm doing this so (student's name) can avoid doing work in class."
Student: Hey! That's... I'm offended.
Me: And I'm offended that you would use your religion to justify your laziness. So I guess we're even.
Me: Now, get to work.
She did, however grudgingly, go to the board.
*I take the words from SAT prep vocabulary lists.
**This is one of the things I'm known for. Everyone who takes my class knows in advance that they will be going to the board a lot.