There are many solutions that involve editing the Joomla Core files. This is bad idea, because you will have to do this every time you upgrade Joomla to a new version. In general changing Joomla core files is not recommended, and you should avoid it as much as possible.
Solution that I find to be much better and cleaner is just to add :
$this->setGenerator(‘Text you want to display in generator tag’);
before <head> tag in your template index.php file.
This way you can define your own generator, or you could leave the text empty.
Note: Don’t remove or edit this tag if you don’t have the strong reason to do so. We should all support Joomla, that is the only way Joomla will keep being great CMS as it is now