You can unlink Amazon Alexa Voice Service on your ecobee4 thermostat, through the ecobee mobile app or through the ecobee web portal.
From the mobile app, select the ecobee4 thermostat and then tap the microphone icon on the home screen. Select UNLINK. On the web portal, select AMAZON VOICE SERVICE then select UNLINK.
To re-link Amazon Alexa Voice Service, you can select LINK and simply re-enter your Amazon.com account information.
If you would like to unlink your Amazon.com account from your ecobee4, go to your Amazon.com account page. Click on MANAGE CONTENT AND DEVICES. Select DEVICES. Choose your ecobee device and select the action button. Now select DEREGISTER.