Follow below steps to deregister / remove device such as Echo, Fire TV, or Fire tablet from Amazon Alexa account if you no longer want to use it to purchase apps or in-app items:
Step 1: Login to amazon.com.
Step 2: Hover over Accounts & Lists and click Manage Your Content and Devices.
Step 3: Click to open Your Devices tab.
Step 4: To the left of each device is a …* button.
Step 5: Click on …* button for the device you wish to remove and click Deregsiter.
Step 6: You’ll get a pop-up warning saying that doing this will remove all content from the device and many features will not work. Click on Deregister to confirm the action.
Note: Deregistering device prohibits Appstore purchases on that device, and disables the use of any app purchased from Amazon Appstore for Android and downloaded to that device. You can re-enable purchasing from the device by signing in to the Amazon Appstore for Android again.