How to prevent creation of subfolder?

[mash180]

This is how my roots setup looks like:

'roots' => array(
	array(
		'driver'        => 'LocalFileSystem',
		'path'          => '/MainFolder',
		'winHashFix'    => DIRECTORY_SEPARATOR !== '/',
		'uploadDeny'    => array('all'),
		'uploadAllow'   => array('image/png','image/jpeg','image/jpg','image/bmp'),
		'uploadOrder'   => array('deny', 'allow'),
		'accessControl' => 'access',
		'uploadTotalSize' => '50M',
		'uploadMaxSize' => '2M',
		'defaults'		=> array(
			'read'   => true,
			'write'  => true
		),
		'attributes'	=> array(
			array( // Hide _*
				'pattern'	=> '/\_tmb/',
				'hidden'	=> true
			)
		),
		'tmbPath'		=> '_tmb',
	)
)

My users should create subfolders inside the MainFolder, but I need to prevent folder creation inside this subfolders. I already try several patterns, for example:

'attributes' => array(
	array( 
		'pattern'	=> '/^.+/\^.+/',
		'locked'	=> true
	)
)

And I try to modified mkdir like:

'bind' => array(
        'mkdir.pre' => [
        	function($cmd, $result, $args, $elfinder) {

        	}
        ]
),

but I can not understand how to work with $args or $elfinder variables, and I'm not really sure about using this aproach.

Can you please point me to the right direction?

Thanks in advance.