Currently it's not possible.
This was suggested some time ago to our team
, but still, we doubt implementing that. This feature can actually lead to the claims from the user side, because in some cases, users want not just to delete their account data, but their posts and topics as well. When deleting an account, we must take into attention that some of the add-ons data like Avatars, Private Messages and many other stuff related to the users, should be deleted as well.
I have an add-on which I've programmed for one of our customers in the past, so if you hardly need this feature, so far I could suggest you to contact us privately and discuss the price of it. I doubt we will include it in miniBB by default, because it's requested rarely, and it means losing benefits of controlling the board.