Airline Club

One thing you might be able to do, if you're able to send out mass-emails to the addresses utilized during registration (or can implement the ability to do so), is create an automated process which, after an account has been inactive for a certain period of time, sends out an email warning of possible suspension/bankruptcy/whatever. You could potentially set it up to only affect accounts after x number of days after an email notification or after x number of email notifications sent at certain intervals. Imposing a forced bankruptcy after, say, six months of inactivity is far less unreasonable if every three months of inactivity triggers an email reminder saying "hey your account might get deactivated if you don't log in once." Maybe something to consider as an alternative if you can't get the freeze to work quite right, or as an additional measure to be used at the extremities (maybe to purge accounts that are inactive for like 2+ years or something).

Problem is, people who are on hiatus usually have a reason IRL to go on hiatus. This game could likely be the furthest from their mind at the moment that email was sent to them, so you're going to see a lot of people ignoring these emails. And then we're back to the problem of coming back after X months/years once the reasons for hiatus are gone and finding the airline forcibly purged. Generally, any solution that purges airlines I disagree with.