With this patch to blocks.inc.php:

— include/blocks.inc.php 2011-09-28 11:13:58.000000000 +0200
+++ include/blocks.inc.php.new 2011-10-06 09:01:46.000000000 +0200
@@ -324,6 +324,7 @@

foreach($groups as $gnum => $group)

{ + $group_display = 0; $group_row = new CRow(); if(is_show_all_nodes()) $group_row->addItem(get_node_name_by_elid($group['groupid'])); @@ -440,8 +441,9 @@ $group_row->addItem(new CCol(array($unack_count, $trigger_count), get_severity_style($severity, $data['count']))); break; }

+ if ($data['count']) $group_display = 1;
}
-
+ if ($group_display == 1)
$table->addRow($group_row);
}

@@ -758,6 +760,7 @@
break;
}

+ if ($problematic_count)
$table->addRow($group_row);
}

You can only display rows in hosts and groups which have problems