Once you've started creating reminders, you'll also start marking them as complete once you've finished with a task. If there's ever a time that you've created a reminder for a task that you later don't need to perform, you could always mark it as complete to get rid of it, but there's actually a better solution. The Reminders app also gives you the option to delete it.
Launch the Reminders app from the Home screen of your iPhone or iPad.
Find the task that you need to completely delete.
Swipe left or right on the task or item name.
You'll now see a delete button appear to the right of the task name.
Tap on Delete to remove it from the list completely.
The task will now be deleted instead of moved to the completed section as if you were to mark it as complete to get rid of it.