Delete Caldera Forms entry after the message is sent. caldera_forms_submit_complete hook. See: https://calderaforms.com/doc/delete-entry-sent/

caldera_forms_submit_complete-delete-message.php

<?php

/**
 * Delete a message from database once submission is complete
 *
 * Deletes from Caldera Forms Pro, which needs local DB storage to send.
 */
add_filter( 'caldera_forms_submit_complete',
	function( $form, $referrer, $process_id, $entry_id ){


		//Delete from Caldera Forms Pro if API client is active
		if( defined( 'CF_PRO_VER' ) ) {
			$api_client = \calderawp\calderaforms\pro\container::get_instance()->get_api_client();

			//requires v0.10.0+
			if( method_exists( $api_clint, 'delete_by_local_id' )){
				$api_client->delete_by_local_id( $entry_id );
			}
		}

		//Delete in local database
		Caldera_Forms_Entry_Bulk::delete_entries( array( $entry_id ) );
	},
	//Keep priority above 50 or email will never get sent.
	99, 4
);