From any device currently linked to your account, go to Settings.
- On a Desktop like OS X or Windows, click the orange arrow by your name on the left side of the application window.
- On a mobile device like iOS or Android, click the menu symbol on the upper left, then click Settings -> Active Devices.
- On older versions this section may be under Settings -> Account.
From here, you'll see a list of devices activated on your account.
From this menu you're able to click the "X" next to any device to remove it from your account.
Once you remove a device from your account, if you want to use it again, you will need to logout and then use the “Reset Your Device” option to be able to send and receive messages again.