And specifically, all you have to do is comment out the following lines in qcodo.inc.php:
QApplicationBase::$PreloadedClassFile['_enumerations'] = __QCODO_CORE__ . '/qform/_enumerations.inc.php';
QApplicationBase::$PreloadedClassFile['qcontrolbase'] = __QCODO_CORE__ . '/qform/QControlBase.class.php';
QApplicationBase::$PreloadedClassFile['qcontrol'] = __QCODO__ . '/qform/QControl.class.php';
QApplicationBase::$PreloadedClassFile['qformbase'] = __QCODO_CORE__ . '/qform/QFormBase.class.php';
QApplicationBase::$PreloadedClassFile['qform'] = __QCODO__ . '/qform/QForm.class.php';
QApplicationBase::$PreloadedClassFile['_actions'] = __QCODO_CORE__ . '/qform/_actions.inc.php';
QApplicationBase::$PreloadedClassFile['_events'] = __QCODO_CORE__ . '/qform/_events.inc.php';
Once those are commented out, you've essentially “disabled” qforms.
Although, just a head's up -- if you are using a PHP opcode cache (e.g. APC) which would be highly recommended on any application that you are trying to improve performance for, then there's actually little need to do even this. If you simply don't call a QForm object, it will never get used. And of course having the code on disk has no affect on performance either.