Hi @IRayner,
It’s not possible to set a contact to marketing then back to non-marketing in the same month. (Don’t shoot the messenger.) This is unfortunate, yes, and ideally HubSpot should not allow setting bounced contacts to marketing contacts. For the current month, there isn’t anything you can do.
In the future, I’d recommend using the contact index page’s filters (Menu > Contacts > Contacts) to set contacts to marketing contacts after excluding hard bounces, unsubscribes, invalid email addresses, contacts without email addresses.
Best regards!