Just keep a naval blockade around the islands and starve them until they decide to surrender.
Supposedly there were back door negotiations to end the war and thereby allow Japan to save face. Had that happened there would be no invasion and no bombs. Negotiations would not have settled the issue. It would have only offered a pause until the next round of fighting starts. Just look at Korea.
It’s the same as a legal settlement where no one admits guilt when everyone knows who the guilty are. Japan was very guilty.
Why? Payback is magnificent and everybody wanted to go home.