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do you mean you want your center l and r, center/center blocks and your content to be full width, no blocks on left side? if so you will most likey have to edit your theme.html
edit: if you want your forum page to be full width you can do this from system admin -> blocks. you would set it up so your blocks only appear on start page. you could add more links to nav bar, like login if you make forums your start page.