You can unregister your device in two ways: first, from your mobile phone, and second, from the Amazon website.
To do so, just follow the tutorial below.
How can I delete my Alexa account?
Method 1 (Alexa App) For Android, iPhone
1. Open the Alexa app on your phone.
2. Tap the menu icon in the app's upper left corner.
3. Select the Settings option.
4. From the settings screen, click on the device settings option.
After pressing on the device settings, you will see all the devices that are registered with Alexa.
5. Click on the device for which you want to de-register.
6. You will see some information about that device. After pressing on the device option, scroll down, and after the registered to option, press the deregister option.
After following the steps, your device will be fully deregistered from Alexa's apps.
Method 2 (Alexa Website)
1) First, go to Alexa's website and log in.
2) From the Alexa web site's sidebar, select the Settings option.
3) After clicking on the settings, you will see the device names. Click on the device you want to de-register.
4) You will then see all of the information for that device; scroll down and select the option to deregister the account.
1) How to Deregister Amazon Echo Devices?
Ans. To deregister a device from the Echo Devices, open the Alexa app > tap on the menu > Settings > Devices Settings > select the device that you want to de-register > tap on de-register.2) How can I delete my voice history from Alexa?