Yes :) How will you control a user that you have never seen, but he has his own forum in your forum. Impossible. Noone allows sich thing in his house :)
But OK, you are liberal. Than you can simply make 4-5 basic forums for various areas and tell: "Here they are, create your own topics and discuss whatever you want."
There is also posiibility the user to close the topic he has created. You'll have also options the user to delete the topics and/or the messages he has created... All this makes the said user like a mini-admin of his topics. (If you allow him to be.)
Anything more is... hm... no good ;)