As far as I know, there's no way to do that in 1.9[1]. Either you are enrolled (and can access the course) or you aren't.
Saludos. Iñaki.
[1] 2.0 has changed a few things regarding enrolment, and I don't know if this would be possible or not.
Enrolment
Deny access to some users and not to some few users
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