I tried to uninstall Admin Tools. I used Joomla Backend, Manage Extensions and uninstall "Admin Tools package". to uninstall all of the component, plugins, etc.
There appaeared a message:
Failed to delete /plugins/system/admintools/admintools
So I tried to use FTP to delete it. Message:
550 RNFR command failed.
550 Remove directory operation failed.
Even to rename the manin-php to main-disable.php failed.
So I installed Admin tools again. Now I can rename the manin-php to main-disable.php. But it is still not possible to delete it with FTP.
I recognized the same problem with another joomla test installation. I can delete manually all of the joomla directory, but not /plugins/system/admintools/admintools