Hi @lucy,
There are two levels in user deactivation/deletion:
The first one is user deactivation which is what you performed. After deactivating a user, that user becomes archived and you can still search him/her and read their messages in channels.
The second is user deletion. You can do this via a CLI command. Note that when you permanently delete a user, all their posts and past activities will get permanently deleted.
Hope this helps,