[kh99]

You could enable human verification for posting, then disable it for all but one forum by creating this plugin using hook location global_setup_complete:

if ((THIS_SCRIPT == 'newthread' AND $foruminfo['forumid'] != 2) OR
    (THIS_SCRIPT == 'newreply' AND $threadinfo['forumid'] != 2))
{
    $vbulletin->options['hvcheck'] &= ~($vbulletin->bf_misc_hvcheck['post']);
}

of course, change the 2's to the forum id where you want CAPTCHA.