In this blog it is explained that how can turn off security permission of parent site. There are following steps.
1- Navigated to sub site, click Settings, and then Site Settings.
2- On the Site Settings page, under Users and Permissions, click Site Permissions.
3- On the Permissions Tab, click button stop inheriting permissions.
4- Select all parent site group which you remove permission to this sub site, click to Remove User Permissions to selected group.