3
^^exactly.
More hints: if you have any blocks that are set to "All Pages" you will still have that block appearing. You'll have to deselect that option and then select the (multiple) pages for your blocks to appear where you want them. You might want some navigation buttons in your header so people can still get around with just a click.
So in your case you shouldn't have any left/right blocks set to newbb if you want it full screen width.
This brings me to a wish list item. I'd love an "All pages except xxx" option for blocks. Side effect of no All Pages option is that root pages like user.php and register.php look ugly full screen. I have a theme trick to fix that but is a workaround.
Also look into other hungry tables like _priv_messages. I've seen some very large PM tables. It can get sensitive asking your users to clean up, yeah right...