The best way to begin is to login as that user and use the Alert Plus Subscriber Web Part to get the list of alerts the employee is subscribed to. Check the recipient type options for these alerts and delete all alerts that the user created selecting “Send to Me”. If they are part of a “Send to a Fixed Set of Users or E-Mail Addresses”, then edit those alerts and remove them from the recipient list. Alerts that are based on a Contacts list, site group, or cross-site group can remain, but the departing employee should be removed from the source group. Alerts with a lookup recipient type will need to have the source column for the e-mail address reset to another user.