Disable the notification bar and the external buttons

Hello, I need help in the development of my app, I need that my app be always on top like the payment module, when the poynt system request for the card swipe and blocks everything, like the home button, volume and block key, and the notification bar.
Its because my app needs to be launched just after the payment and my app needs to be always on top and close only when the user press the cancel button or fill the form.
How can I do this?