The Atto editor turn on spellchecking when it initializes here. You can change this value to false, install and run shifter in atto/yui/src to compress the changes. This will disabled spellcheck wherever Atto is used which may not be what you want.
It would be better to create an Atto plugin to change the value after the editor loads on essay questions.
I would like to disable spell checking only in quizzes not for the whole site.
Is there any doc about plugin development? Maybe I can try to do something.
Atto plugins are some of the easiest to start on. Documentation for creating plugins is available Atto docs. Your plugin would be very simple. It does not need to add a button, but only set attribute of the editor in the initialization code.