Want to turn Regisration on on Sub Site and leave registration off on Main Site

Here is one I have been searching today burning up Google's and Bing's servers to find a plugin or answer for. We all know that in order for sub site to have registration on the main site has to have Registration turned on weather you want it or not.
Now what happens if you don't want the Registration to be on in some sub sites and main site. But need it on in one Sub site.
Looking for any ideas or plugins you know about.

My head hurts :-<