During any stage of the Workflow, you have the option to 'Delete' or 'Blocklist' one or more opportunities.
When an opportunity is deleted, it is removed from the campaign but it can be discovered for future campaigns.
When you add one or more opportunities to the Blocklist (Global or Project-specific), it will be removed from the campaign and will not be pulled in for future campaigns.
For more information about Blocklist functionality, you can check out the article here!
If you are an Admin, you will have the option to add to the Blocklist by clicking 'Settings' then 'Prospecting Blocklists' at the top of the screen.
From this screen, Admins can review all sites added to the Prospecting Blocklists. They also can add or remove opportunities in bulk.