There is functionality in Asset Bank that automatically suspends user accounts if they haven't logged in for a certain period of time. Suspended users do not count towards your total user limits, and they can be re-activated later should the need arise. 

When active, the user who has not logged in for a specified period of time is sent a notification email stating that they must click a link in the email within a specified number of days to prevent their account from being suspended. This functionality is designed to work for Asset Bank user accounts only and does not work with remote accounts such as those that have been synchronised with Active Directory and use Single Sign-On.

To enable this functionality navigate to Config > System > User Management Settings.

Set 'Auto Suspend Inactive Users?' to Yes, and specify the number of days of inactivity prior to the email being sent, and the number of days they then have to click on this to keep their account live.

Users who have not logged in for more than 1.5 years will be auto-suspended

Please note that to avoid email blacklisting, any user who has not logged in for more than a year and a half will automatically be suspended when this feature is activated. This is to avoid sending emails to accounts that may no longer be live, resulting in high bounce rates.

So, for example, if you set the period of inactivity to trigger the notification email to 180 days (six months), then everyone who has not logged in for between six months and one and a half years will get the notification asking them to click a link to keep their account open. Users who have not logged in for longer than this will just be suspended.
 

If you wish to change the copy of the reactivation email forwarded from Asset Bank, go to Config > Content > Email Templates and select the 'user reactivation' template.