You can't. That's my straightforward answer but actually you can let yourself receive less of these junk mail. Choose an email client that has a great spam filter!

I have been using Yahoo Mail for six years now, and what I notice is the spam messages I receive in my inbox. I even remembered one time, that I got disapponted because I didn't have any bank account information to give to claim a prize, or to help elderly people transfer their wealth to me. But that was the time I was still ignorant girl. But anyways, after signing up a Gmail account, I have never worry again of receiving junk mail. I don't claim that I stop receiving them, but I have just seen them once in a while in my Gmail inbox. Because what Gmail does is, when they suspect that an email is a spam, they automatically put it in your spam folder. And all spam more than 30 days old will be automatically deleted.

Some emails though can get through, and sit there in your inbox. But you don't have to be annoyed when this happens. All you have to do is check the box that corresponds to the email you think is a junk mail, and then click the "Report Spam" button. The next time you will get a similar message again, or the next time you will receive another message from that sender, Gmail will put his email in your spam folder and not in your inbox. So, you don't see spam at all, unless you check your spam folder.

I rarely use my yahoo mail account now. But I still check them once in a while.