Spam is a big problem in the modern email world, but there are a lot of different ways you can both reduce the amount you receive and deal with the spam that slips through the cracks. Yahoo! already has fairly powerful anti-spam filters, and any messages you find in your Spam folder can be safely left there. With good email habits and regular reporting, you can minimize the amount of spam you have to deal with.
Not all email advertisements are spam. If you've signed up for a newsletter, or left a box checked when creating an account that signs you up for special offer emails, these messages are legitimate advertisements and can be dealt with quite easily. https://www.spamtitan.com/hosted-anti-spam-comparison/
If you are repeatedly receiving messages from an address that is bypassing Yahoo!'s spam filter, you can permanently block it. Note that in order to block addresses, you'll need to log into the Yahoo! Mail web interface on a computer; you can't block addresses from your mobile device's Mail app.
Most of the marketing and junk email that fills up your inbox are actually perfectly safe marketing messages. You can unsubscribe from these messages by following the "Unsubscribe" or "Email preferences" link at the bottom of the message.