The hard bounce status might be stored outside of your lists. I am not sure customers can easily change a hard bounce status themselves. Do you mean you just deleted those records with intent to re-add to reset the status? On our BigMailer platform this wouldn't work as hard bounce status would get preserved.
We use SendGrid and Mailgun right now, and both of these expose the suppression list, email address, time, and reason code + description. In Sendgrid you can filter, and mass select to remove suppressions easily (which was great). In mailgun I had to export a CSV and just removed them manually as there was not too many across my accounts.
Customers generally cannot change this on their end as far as I can imagine -- this is on the ESP end and is a protection built in because you are sending from their IP / Server and they don't take kindly to that.
I logged into our sendgrid and mailgun accounts and manually purged all the failed gmail records.