Hello Steve,
Administrators can delete posts at any time by clicking "delete" at the bottom of the post. If the post is in a thread, I think all the following posts have to be deleted as well (or possibly moved to a new thread), not sure.
Administrators can edit posts by others very nicely in 1.5.3+. If you check the config-dist.php file, line 153 contains this:
// $CFG->admineditalways = true;
Paste that line in the new config.php file and remove the slashes so it looks like this:
$CFG->admineditalways = true;
Then, the administrator will see a new "edit" option at the bottom of each post, and after editing a post, there will be a little note following your edit within the edited post that says something like, "Edited by NAME on January 1, 2006."