Right.
First thing is to add a group say called 100 club .Set the post count to what you want 100.
The goto the forum permissions and set the permissions to post and reply to allow only 100 club. Or for that matter you can completely hide the forum from any group you want
When a member reaches 100 posts that member will be added to 100 club automatically and still be in whatever groups they were in to begin with.
So whatever permissions 100 club has they will be upgraded per say.
You could do a lot of things like allow bigger attachment sizes add other permissions they did not have etc.
Then you can do the same for say 200 club
