This is indeed possible. When synchronizing users between JIRA and Confluence, Confluence will pull all user, but only users with "can use" permissions will be allowd to use Confluence (and will count against your Confluence license). You will need to make sure that only the users allowed to use confluence have "can use" permissions.
For example, you could have a group called "confluence-users" and have in this group only the users which are going to use Confluence. Then, from Confluence's Global Permissions screen, make sure that the other groups have no "can use" permissions.
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