[kh99]

Create a new plugin by going to Add New Plugin under Plugins & Products in the adminCP. Choose the hook location from the dropdown menu, enter a title so you'll remember what it does later, and paste the code in the large text area. Click the "Yes" radio button to enable it, and press "Save".

To have one forum be exempt, put this around the code:

if ($forum['forumid'] != X)
{
// insert code here
}

and of course replace X with the forum id.