// Unset unwanted Scripts - by name
$unset_scripts = array(
	'k2',
	'...',
);

foreach($this->_scripts as $name=>$script)
{
	if (!preg_match('#(' . implode('|', $unset_scripts) . '#i', $name))
	{
		continue;
	}

	unset($this->_scripts[$name]);
}

or if you use joomla v3.8.1 you can try this php code:

<?php
	$doc = JFactory::getDocument();
	unset($doc->_scripts[JURI::root(true) . '/media/system/js/mootools-more.js']);
	unset($doc->_scripts[JURI::root(true) . '/media/system/js/mootools-core.js']);
	unset($doc->_scripts[JURI::root(true) . '/media/system/js/core.js']);
	unset($doc->_scripts[JURI::root(true) . '/media/system/js/modal.js']);
	unset($doc->_scripts[JURI::root(true) . '/media/system/js/caption.js']);
	unset($doc->_scripts[JURI::root(true) . '/media/jui/js/jquery.min.js']);
	unset($doc->_scripts[JURI::root(true) . '/media/jui/js/jquery-noconflict.js']);
	unset($doc->_scripts[JURI::root(true) . '/media/jui/js/bootstrap.min.js']);
?>

if you wand disable all js & css scripts - use code:

$doc->_scripts     = null;
$doc->_script      = null;
$doc->_styleSheets = null;