Disable DetailsView CommandField

Hi All,

I have a two public variables on a web page. One is a count of prerequisite courses and the other is a count of courses that a user has taken which match the prerequisite courses. What I'm trying to achieve here is if the users count is less than the prerequisite count then there is a course which the user needs to take prior to being allowed to book the course they are viewing, which the information is held in a DetailsView. The CommandField (Select Button) is the command that allows them to book the course, so I need to disable this button if the above situation occurs.

Any ideas how I would achive this?

Thanks.