UBB.threads PHP Forum Software Community Forums Resources and Customizing How Do I? What do I have to do to have a forum where only users with 200 plus post can post?

You make a group that has permissions to the forum.
Then in the group there is a field to set how many posts to auto join the group called post count.
Control Panel » Group Management
It is only editable if it is a not stock group.
In fact at the bottom of that screen it states
"By filling in the Post Count field you can make it so users will automatically be added to that particular group once they reach the designated Post Count. Leave blank, or 0, to disable this feature. "