Currently there is no way to totally remove an existing user if such user has already created some content.
The alternative of deactivating the user addressed this problem just partially, those users can still get listed publicly, they are still somehow around the system.
This turns into a security issue.
When working with groups/users/permissions the inactive users listed and available in the search.
When working with permissions, it would be helpful if inactive users were not listed or available when selecting individual users (i.e. need to make a user active before it can be selected for use). The inactive users should not be shown in the search list of the user while adding a new user in the group.