Can be administrator directory effectively renamed (for example by simple change in Joomla! core code) for example to admTZ34? The main reason is to obfuscate standard pattern where www.example.com/administrator could be used to check presence of Joomla! installation (in older releases also Joomla! version etc.)
It's not really recommended or very simple to do. The admin path gets hardcoded in a lot of extensions with calls to things like JTables which means changing the directory name could have a lot of consequences.
That said if you want to prevent direct access try with one of this solutions:
A) Password protect /administrator with a .htaccess password protection.
This uses HTTP authentication to prevent anyone from getting to the admin directory.
B) Require a keyword in the URL with an extension like:
You can't really change it as it would prevent extensions from installing properly. You can however use Admin Exile which allows you to add a value, key or both to the administrator URL. In addition to this, if the standard admin URL is typed it, it will redirect the user to a site of your choice.
Hope this helps
If your only concern is bad-bots identifying your website as joomla website then you need to do a lot more to hide this truth.
A lot of technique exist to identify your website as joomla and its version. This .htaccess file helps in these attacks.