The iOS updates can be auto-pushed (but never auto-installed) by Apple to the device if:
1. The device has the necessary free storage space for the update.
2. The device is connected to a valid Wi-Fi network
3. The device is connected under external power (connected to a PC via USB or connected to a USB charger)
If you don't want to install the update, you don't have to. If you choose not to install the update, delete the update file to free up the storage space that it uses. Do this under: Settings/General/Usage/Storage/Manage Storage. It will appear in this list and can be deleted by tapping on the "iOS update" app, then tapping the "Delete App" button.
Beware that the update may be auto-pushed to your device again in the future so if you notice storage space missing, this may have happened.