This makes it somewhat different from the student loan question as I believe you would need to write a check to the U.S. Treasury as a citizen donation to offset it. Is that how others are reading the subsidy mechanics?
I believe you are correct as far as the ACA subsidies work. It's going to be a refundable tax credit somewhere around line 64-65 on page 2 of the 1040. Just like the additional child tax credit and the earned income credit. That's my speculation at least, but it is a refundable credit.
As far as distinguishing the ACA subsidy from the IBR, it is virtually the same. Under IBR, you have to certify your income annually. This means mailing in a copy of your form 1040. They take the AGI off your tax form, and the number of personal deductions you have, and the simple formula pops out what you owe for the next 12 months.
In order to avoid having your IBR payments lowered, you would have to intentionally fail to send them your 1040 each year. The payments would then default to some crazy 10 year amortization (from 30 years in my case) and in my situation I would owe over $1000 per month.
And come to think of it, I think I signed a form giving them permission to get my tax forms each year from the IRS, so the annual re-certification might be automatic.
I suppose one could exit the IBR program altogether and go back to their old payment plan to avoid very low or zero dollar student loan payments. Similarly, you can buy non-exchange health insurance in the private market which would disqualify you from receiving any subsidies (only way to get subsidies is through the exchange).
Ethically, if you disagree with the IBR program being used by early retirees to potentially avoid some repayment of debt, you should be of the exact same opinion regarding low income and ACA subsidies. IBR involves reduction of a payment amount due to debt incurred to provide a service (education) and ACA involves reduction of a payment amount due to provision of a service (health insurance).
Under both programs, one uses a low income to qualify for large monetary subsidies from the government. If the ethical requirement is that one should keep working to repay student loan debt in full, one should also continue working to afford full price non-subsidized health insurance.