Generally people want to keep the email for users that have left the organization. I'd advise against doing this.

You can delete messages from the archive server using the search plugin as long as the container file (.emx) if not written to WORM storage.

Depending on the number of users in the company it may make more sense to create separate collection folders for individual users. You could delete the folders after they left. The downside to this approach is that you'll lose single instance storage on the archive server.