This is very good information to have guys! Be sure to follow this and set this up so you will have a step up on any attempts to hijack your accounts!
Some time ago, Valve added a new feature that allows account owners to lock their own accounts if they suspect that they've been hijacked.
A link to lock your account is provided in Steam Guard emails that are sent when you try to login from a new device. I'd recommend that you trigger a Steam Guard email to get that link and bookmark it if your account is even slightly valuable.
Clicking that link will take you to a page https://i.imgur.com/701cfMZ.png with a big green button that you can click to lock your account. This page is accessible even if you're unable to login to your account.
Clicking that button will instantly lock your account and prevent it from purchasing things in the store, trading, using the Steam Community, playing online on VAC-secured servers (potentially all servers?), activating CD keys, and gifting items. Once locked, you can't unlock it without proving ownership to Steam Support.
Since you have to go through Steam Support to unlock your account and it's basically useless until then (even if you regain access to your account, you can pretty much only play single player games. You can't even change your profile name), I'd advise only using this button as a last resort when you're sure that you've been hijacked. If your password hasn't been changed, I'd advise first changing your password and email address.
You can trigger a Steam Guard email easily by logging into Steam from an incognito/private browser window.