How can I stop autocorrect in a granular way?
When you type something several times and ignore the suggestion, iOS assumes you mean to type it that way and adds the word to the Keyboard Dictionary. You can reset the dictionary by going to Settings > General > Reset and tapping "Reset Keyboard Dictionary". You cannot reset the dictionary in a granular way.
A possible workaround, however, if you don't want to reset the whole dictionary, is to add the misspelling as a shortcut and set the replacement phrase as the word spelled correctly. To do this go to Settings > General > Keyboard.