When you're ready to publish the job on your careers page, click the option to Publish in the job editor. Then, access the Find Candidates tab in the editor.
Click the Free Job Boards option and you'll find a link to 'Unpublish from all free boards'. Click this link and the job will not be published to the boards (or it will be removed from them immediately if it's already published).
The status of the job will update to read "Published (Careers Page Only)". You can republish to job boards at any time through the Find Candidates section.