There are two places you manage contacts that you do not want to reach out to: your Do Not Email list and your Prospecting Blocklist. Both can be used to prevent outreach to certain opportunities, but their functions are slightly different.
This list focuses on opportunities. When you add an opportunity to your blocklist, the site will no longer be prospected when running a new campaign. They will no longer appear in the inspect stage of the workflow to ensure that you do not reach out to them. Sites can be blocked project-specific or account wide, depending on your preference. You can see the sites that have been filtered out during prospecting due to the blocklist on your prospecting summary.
Visit this Knowledge Base article to read in-depth about adding and removing sites to your Prospecting Blocklist.
Do Not Email
This list focuses on contacts - it is an unsubscribe list. Contacts can be added to this list by replying to your outreach with the word "UNSUBSCRIBE", or you can add them to the list from the opportunity details page.
Contacts added to the unsubscribe list may still appear in the prospecting stage. However, when the email template sequence sends out, the system will prevent this contact from being outreached and move on to the next.
Visit this Knowledge Base article to read more about your Do No Email list.