In the index.template.php of your SMF template you have below <body> the header of the forum. Now I only now for sure how to remove the entire header, but you want to get rid the thing in the header. So find it (I think comment of that part goes like this:
// Otherwise they're a guest - this time ask them to either register or login - lazy bums...
So remove below and to the next part, that should do it.
But since Iam not completly sure, back up your file beforehand
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