[peterska2]

You can do this via usergroup permissions.

Create a new usergroup based of the registered users usergroup and call it members. Then create a promotion (usergroups > promotions) to promote from the registered users usergroup to the members usergroup after one post.

Then in the forum permissions manager, deny permissions to post in all bar one forum for the registered users usergroup.

Make sure that you post and announcement stating that after their first post it may take up to one hour for them to be able to post in the other forums, as the cron job that does the promotions runs hourly.