In the block under "Show block on specific pages:":
Select --> Show if the following PHP code returns TRUE (PHP-mode, experts only).
Enter the PHP code:

<?php
$match = TRUE;
if (arg(0) == 'forum') { $match = FALSE;}
if (arg(0) == 'node' && ctype_digit(arg(1))) {
$node = node_load(arg(1));
if ($node->type == 'forum') { $match = FALSE; }
}
if (substr($_SERVER["REQUEST_URI"], 0, 6) == '/admin') { $match = FALSE;}
return $match;
?>

More: http://drupal.org/node/64135