The recommended way of retiring user accounts is to disable them. Scenarios where this is useful is when a person leaves the team and it is necessary to retire their account.
Once an account is disabled the following will be enforced:
All currently active sessions for the account will be invalidated (i.e. automatically logged out).
It will no longer be possible login using this account.
No further email notifications will be sent to the account once it is disabled.
The user account will not show anymore in lists like "assign to", "send reminder to", etc.
The disabling process is totally reversible. Hence, the account can be re-enabled and all the account history will remain intact. For example, the user will still have issues reported by them, assigned to them, monitored by them, etc.