When someone leaves the company or may be scheduled for a long term leave of absence, you can Deactivate their User Account. It is also possible to Delete User Accounts but only if the user has never even logged into the system.
In either case, you will be prompted to Reassign any possible pending activities for the user you are deactivating or deleting. The purpose of the Reassignments is to ensure that any workflow tasks pending for the user are not left hanging. Even though the user may have no pending tasks, this process currently requires you to think about the reassignment of all possible tasks. You can reassign them to someone who is replacing this user, the person's manager or to yourself as the System Administrator. Some clients actually create a fake user for this purpose. In that case they use the same email address as the System Administrator use a fake name starting with 1 or with A so that it is conveniently at the top of the User List during the reassignment process. In any case, any Notifications related to outstanding tasks will now go to someone else so they can be taken care of.
You will also get an alert if the person you are deactivating or deleting is the last person with a particular Function Role assignment. This won't prevent you from the deactivation or deletion of the account but alerts you in case that Function Role is associated with things such as document permissions, document workflows, training activities, etc.
Once a user is deactivated, they will show up on the Deactivated User list available to System Administrators. There is an option to Reactivate the user if they return from the leave of absence or are possibly rehired in the future. Once a user is deleted, that account no longer exists.
For more information on User Management, refer to SOLABS QM 10: User Guide for System Administrators - Managing Users, Roles and System Configurations.