Requiring a certain amount of posts before one can post a thread

[Snapple]

For vBulletin 4.2.2, I thought I remember having a way to require a certain amount of posts before a user can post in a forum.

For instance, if <user> has 12 posts, they cannot post in 'X' forum because there is a 15 post requirement.

Further elaboration:
There is a 15 post requirement to apply for forum moderator at the forum I work at, and I want to require that 15 post count in order for someone to post in the forum moderator pending application section.

How can I accomplish this?

Thanks in advance,
Jake