If you accidentally marked a candidate as "spam," their record still exists in Greenhouse Recruiting. To add the candidate back to your list, you'll have to find their record and remove the rejection reason:
Navigate to the Candidates page.
Remove the filters at the top of the page by clicking the X icon next to Active.
Select Spam from the Rejection Reason filter on the left side of the page. The Rejection Reason filter is in the Profile Details section.
If you want to add the candidate back to your active candidates pool, click Unreject.