When a user is no longer active (and should not be able to access your knowledge base anymore), it can be deleted or deactivated.
Deactivating the user will only make him unable to sign in. All tracked activity will be available to be seen.
Deleting him will make it impossible to track his activity afterwards. Therefore, it won't be possible to see what he's done since he was first created. Nonetheless, the questions and categories created by him will remain intact.
As a general rule, we suggest that you only delete a user if he/she is a test user or has never done anything in Helpjuice. If the user was ever active and working on your Knowledge Base, we recommend you to just deactivate it to maintain trackable data.