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.