I assume that you are viewing your Yahoo account via the website and that the unwanted mails are going into the spam folder on that account. You can't stop those emails coming into your account: once your email address is on the spammers' mailing lists they will keep sending it to you. If you install Thunderbird on your computer, and if your Yahoo account is set up to use IMAP, you can tell Thunderbird not to download those messages to your computer: but they will still be there in the Yahoo spam folder, you haven't got rid of them, you have just not downloaded them. Be aware also that the filter used by Yahoo to identify mail as 'spam' isn't 100% accurate, so there is a risk that mail you actually want to read gets sent to the spam folder by mistake. If you never download it to your machine you will never know it's there. The simplest approach I think would be to log in to your Yahoo account via the website, go to 'settings' (the cog wheel at the top right), click on 'security' and set Yahoo to delete all spam after the shortest possible interval, which I think is one week. You can also set it not to show images in the spam folder.

Thank you for the clear information and precise guidance in how to navigate. I was able to learn more about the problem and also make a few security changes.