Good day, @Simone - 

We'd love to be able to assist you, but we'll need your full post in order to do so.  Please reply so we can get you connected up with someone who can answer your question.

-Sam

Once Improvements/Incidents have been added to the portfolio the portfolio can't be deleted.  You can disable a portfolio by using this option:

Some clients will opt to rename the portfolio as "delete" or similar too.  

If you would like to see an option to archive you can make this suggestion on our User Voice page (https://nintex.uservoice.com/)