I would create a different account, setup their old username as an alias to the new account then create a .forward rule to forward any mail that arrives at that address to a personal account they own, or a webmail account at someplace like gmail.
So mail would still be received at their original address and forwarded to an account they can access, yet they wouldn't have login information for the real account. and since it's forwarded to an account they own any replies from them wouldn't come form your company email server..
A bit screwy but it could work...
You can also set an expiry date on the user account so it will expire automatically.