During "Inspection" you can add an opportunity to the Blacklist, by simply clicking the drop-down arrow next to the “Delete” button and select “Project Blacklist”. If you are an admin, you will also see the option to select "Global Blacklist" (for more on the difference between Global and Project level Blacklist you can check out the article here!)
You can also blacklist multiple opportunities at once by checking the boxes next to the opportunities you want to blacklist and hit the drop down again and hit “Blacklist” all of the checked opportunities will be added to your blacklist. If you blacklist a result, it will not show up for any future campaigns in that project. If you simply delete a result, it could reappear in future prospecting campaigns.