You don't say what sort of setup you have? You say you are a small firm, but do you have a network installed? If so I imagine disabling the account in User Manager would be one of my first jobs, depending on you network OS you can even set an expiry for the profile so it expires on the day the guy leaves (in case you forget or are busy)
Some companies allow the person to leave a day early and only tell them at the end of that day, if they are worried about malicious acts on the network.
You could also change the user account to a limited account if this was your concern.
With regards to emails, you can bounce these back to the senders informing them that the person has left, although I would place caution on that, as there may be communication in them that is relevant to another person or someone who is taking over the leavers role.