Could you not go back and just give them an arbitrarily high number, say, 10 years. To the point that everyone will have long forgotten.
You could also try argue say, 5 years from the last point of contact, as banned accounts after that time are likely to never be wanted nor missed, and that you have a legitimate interest in retaining the data for that period. If a banned user frequently contacts you, it suggests they are attempting to undermine the ban from your site and thus you need the data to keep them off your system.
That doesn't really work though does it? Say they email you, the account in XF has no record of that. Short of manually adding some sort of note to an account and somehow keeping track of them all it doesn't work.
We're happy with removing all email addresses and dobs from accounts that have been banned for 5 years. It's still not easy to do this though, even manually.