Enable the Blacklist Honeypot. This feature will make it appear that the submission was successful to the submitting user; however, the submission will not actually be processed.
Apply to all fields on all forms.
add_filter( 'gpb_enable_honeypot', '__return_true' );
Apply to all fields on a specific form.
add_filter( 'gpb_enable_honeypot_FORMID', '__return_true' );
Apply to a specific field on a specific form.
add_filter( 'gpb_enable_honeypot_FORMID_FIELDID', '__return_true' );
This filter is available since Gravity Forms Blacklist 1.2.1.