Control Views Bulk Operation messages?
You can uncheck "Display processing result" in the VBO settings (in the edit view UI).

VBO for D7 stopped generating that many messages in latest -devs (after beta1). It now only shows one message: "Performed $operation on $count items."

That is something that should be backported to D6, there's an open issue for it.
In general, you can't affect those, the module is just calling drupal_set_message (usually in _views_bulk_operations_execute_finished()).