[Paul M]

This is completely untested and unsupported but try this ;

Create a new plugin, linked to the global_start hook and containing this code;

PHP Code:

$xforumid = 123;
$cutoff = TIMENOW - 86400;
$data = $vbulletin->db->query_first("SELECT COUNT(*) AS posts FROM ".TABLE_PREFIX."post WHERE dateline > $cutoff AND userid = {$vbulletin->userinfo['userid']}");
if (intval($data['posts']) == 0)
{
    $vbulletin->userinfo['forumpermissions'][$xforumid] = 0;
} 


Change the value of $xforumid to suit.

Note this should block that forum for anyone, even administrators, if they don't meet the requirement.