It's too bad you're not using v 3.3 of Moodle. It has a really slick new feature in the forum that specifically allows you to restrict access to a forum based on criteria that you can select. One option is to restrict it based on specific keywords in a user profile so you could restrict a specific person based on their name. I was just messing around with this new feature and I can definitely see how I'll be able to use it!
What about this for a workaround in v2.7? Could you use groups and groupings to set up a "no posting for you" group/grouping and then set your forum group mode to separate groups so that only students in the permitted group can see the posts in their group? The student in the restricted group would still be able to post but no one else would be able to see their posts other than the teacher.