In the hands of skilled writers, even the "thinnest" device can be wielded artfully. However, because many of your students are, much like mine, novices, they probably have a habit of "glomming onto" the device that catches their eye at the moment. I have had this issue repeatedly; they want to use a device, a word, even a piece of punctuation they have recently learned, regardless of the task, situation, or audience. It can be both comical and frustrating to see the same devices in everyone's essay.
Perhaps inviting them to brainstorm when/where/how/why a device would be effective...essentially putting it into different contexts and seeing what works for a given topic/purpose/audience. For example, a recent, painful tragedy in a community that is elegiac in purpose may benefit from repetition of a unifying "we," or an uplifting metaphor, but would not be a good time for irony.