How do I deauthorize a device?
Deauthorizing a machine means it will look like a new device next time you use it to log in and a Steam Guard code will be required.
If you've mistakenly checked the "remember me" box when logging in to a public computer or if your account has been compromised, you should deauthorize any computers that you've previously Steam Guarded. You can do this from your Account Details page > Manage Steam Guard and select "Deauthorize all other devices" at the bottom of the page. This will deauthorize all computers or devices other than the one you're performing this action from.