Currently there is no way in BLOX to restrict on a per article or asset basis.  All restriction is done by section/URL (news, sports, opinion etc.). 

One work around for this is to create a section under Settings > Tags > Sections called for example, subscriber_only.

Mark this section/URL as restricted under your service. When creating your article if it goes in your opinion section which is not a restricted section, then assign your new article to both the newly created subscriber_only section and the opinion section. It will still show up on the opinion section page, but when clicked on it will require that the user have the specified service to view the full article since its native section is subscriber_only.

NOTE: Make sure that subscriber_only is the first section listed making it the articles native section.