There is a fast way to delete the spam that is already in the spam folder or trash folder.
WordPress Spam |Trash | Deleting it fast | How to
1. Login and go to Dashboard.
2. Click the Comments section.
3. Click either Spam or Trash in the menu.
4. With one click you will delete it all.
You want to check this at least once a month so it does not get too many comments.