If you need to exclude a particular list, library or site from showing up in search results, then you can set the setting 'allow to appear in search results' to NO. But there is also another way of doing this and is helpful when you want to exclude a particular URL or item from the results using search scopes.
Follow these steps -
1) Navigate to your search scope.
2) Add a new rule.
3) Enter the List URL in the correct text box.
4) Select 'exclude' for excluding these specific results from search.
5) Click OK to save.