To remove a phone from you Athom account you need to go to my.athom.com and log in with your account.
Once you are logged click on your account name. A dropdown menu will be displayed. In this dropdown you will see My phones. Click on this to get an overview off all phones that are connected to your account.
To remove one of these phones, hover over the phone until you see a cross being displayed in the top right corner. Click on it to remove this phone.