It's been two weeks and the data still hasn't been cleared out of the ranking table so I have bitten the bullet and implemented my hack above. Will keep all informed if I come across any side-effects.
Code:
su - zimbra
zmprov gmi problem_child@domain.com # to get <zimbra_id>
mysql
UPDATE zimbra.mailbox_metadata SET metadata='' WHERE mailbox_id=<zimbra_id> AND section='CONTACT_RANKINGS';