Jetpack’s Custom Post Type feature adds custom post types like portfolio , testimonials to your WordPress.com blog. But sometimes this features are included with the theme which you are using on the website and you don’t need that jetpack custom post type feature.
You can disable custom post types by following easy steps.
Visit Jetpack’s settings page
After that visit Writing tab, Under writing tab you will find custom post types. If you want to disable portfolio then just disable portfolio.