I would email them and ask what problem they were trying to solve when they signed up, and what you could do to the product to better solve it for them... because clearly if it is unused, it is not solving anything for them, and they will cancel some day.

Better to get some good feedback from a group of people and build some loyal customers in the process. Yes, some will be reminded to cancel, and you will get some churn. But at your low price point, the value of the feedback from the group you keep will eclipse the few hundred lost dollars from those who cancel.