You can never protect yourself 100%. What you do is protect yourself as much as possible and mitigate risk to an acceptable degree. You can never remove all risk.
I could pose as a Yahoo rep claiming that there's been some sort of fault, and somebody else is getting your e-mail, and we're going to have to remove your account and reinstall it. So what we'll do is reset the current password that you have - and by the way, what is it?