As Airstory is currently free, if you choose to no longer use your account your account will become inactive. After a period of 12 months of inactivity your Airstory account will be deemed 'deactivated' and purged from the system.
Before leaving your Airstory account feel free to edit your profile fields to remove any sensitive information.
If you choose to come back to your Airstory account before your account is purged from the system, simply start using Airstory again (i.e. create a card, work in a project...) and your account will be removed from the 'deactivated' queue.