If you delete a user their challenges and ideas (as well as associated attachments, triages, and reviews) are still available in edison365ideas for other team members to work with going forward.
In order to demonstrate, I've created a test user (Adele Vance) and created several challenges and ideas under her user profile. Here's a challenge that she owns and its associated ideas:
Now lets say Adele left the company...
After deletion, her challenges and ideas are still found on the Home page, under Discover, and for those applicable under the Personal Page (observe I can use her as a filter on the left side below).
From Adele's perspective she is locked out of the application and the SharePoint landing page since she is no longer verified through her SharePoint account.