This filter can be used to prevent Caldera Forms Pro from being used to send emails, when it otherwise would, for a specific form.
Examples
Disable Sending Via Caldera Forms Pro For One Form
Disable Sending Via Caldera Forms Pro For All Forms
More Examples
Where Does This Code Go?
When using WordPress hooks to customize Caldera Forms or other plugins you should not modify the plugin files, or you will lose your changes when you update the plugin. Instead you should create a small plugin to hold the custom code. It's easy, learn how here.
Technically you can add the custom code to your theme's functions.php, but then you will not be able to change your theme and keep these customizations.