You can exclude any forums from the New Posts search results by inserting "&exclude=X", where X is the forum ID number, on the end of the search URL and then bookmarking this link. For example, the default "New Posts" search link looks like this:
A "New Posts" search link which excludes the GT5 Marketplace forum would look like this:
Try out the link above
here. The forum ID number (the Marketplace is 239) can be seen in your browser's URL any time you are viewing that forum, like so:
You can exclude more forums from the search results by simply including more forum IDs in the exclude parameters, separated by commas. You can also use "include" to show only forums you specify in the search results.