How can I log out of Steam remotely?
Solution 1:
You can now log out on all other computers by clicking the "Deauthorize all other devices" button at https://store.steampowered.com/twofactor/manage.
Solution 2:
An alternative to "deauthorizing all other devices" is to simply launch a game you purchased through Steam. It should show a window stating that you're logged in on another device, and you'll be able to choose whether or not to log out of said remote device.