Is there any way to make steam ask for guard every time?
Yes, I know it's weird! But there is any way I can do it from steam files?
For example, by editing the loginusers.vdf?
This should be possible by making sure "Remember me" is not checked when logging in. With that Steam will always need to re-authenticate to login.
And yes, changing the value of "RememberMe" to "0" in loginusers.vdf should force it to need to re-authenticate, or you can just logout of the Steam client.