There are two ways to achieve this:
Solution 1:
Install the Bookmarks Folders Locker/Blocker extensionBookmarks Folders Locker/Blocker extension, if the folder is hidden its content is no longer accessible from the search bar.
Solution 2:
If it is a small number of bookmarks, you can exclude them one by one with Shift+Delete
.
Source: address-bar-autocomplete-firefoxaddress-bar-autocomplete-firefox