add_filter( 'wpcf7_form_elements', 'imp_wpcf7_form_elements' );
function imp_wpcf7_form_elements( $content ) {
$str_pos = strpos( $content, 'name="FirstName"' );
$content = substr_replace( $content, ' autocomplete="both" autocomplete="off" ', $str_pos, 0 );

$str_pos = strpos( $content, 'name="LastName"' );
$content = substr_replace( $content, ' autocomplete="both" autocomplete="off" ', $str_pos, 0 );

return $content;
}