Here's how to remove a sensor from the app:
1.) Open the app to the main screen (the screen with the word "Devices" at the top) and tap the sensor you want to delete.
2.) The next screen will be the time graph. Tap the button in the far, upper right.
3.) The next screen will be the configure screen. Scroll to the bottom and you will find the delete and reset (a.k.a. "Clear Data") buttons.
4) Tap the "Delete" button. You will then have a choice to save the existing data as an archive in the app or export it as a CSV before deleting it if you like, or just delete it. Archives show up at the bottom of your device list and can be viewed and exported, but do not accumulate new data.
If you just want to clear your sensor's data, but keep it in the app to continue recording, please see this article.