Depends on what device you are using. On the iPhone there is not bulk delete option, but you can do it on a Mac and if the iPhone is using the same Apple Account, it will then sync with the iPhone and delete them there.
On the Mac, you can go to the Mail app and choose Settings from the Mail menu. After that select Junk Mail. Depending on your Mac OS version, there will either be a tab to see your blocked contacts or a button to press that will open them in the Privacy & Security menu. Once you get there click the first contact and Shift-Click the last and all in between will be selected, so clicking the minus button on that page will delete all of them.