I formulated the question entirely enough in the title. Below I give some background to explain why do I need to do such a crazy thing.
I have a son of 17 years old who has autistic problems. It is hard enough by itself. A part of this problem is that he cannot write fast enough. He can neither keep up with his school mates nor fulfill his tests during the school hour. On the keyboard, however, he types fast enough.
I agreed with his teachers that he will visit the lessons and tests with his laptop. It is understood that he will have an editor. This one should be convenient to type both texts as well as mathematical formulae for the lessons in Mathematics. The teachers have nothing against that.
However, they warned me that in 1 year my son has to pass exams. Such an editor can also be allowed for the exams. However, it will only be accepted, provided the function of the Check Spelling is definitely switched off. It means that it will be impossible ever to switch it on. My son must be unable to switch it on during the exam, check the spelling, and then again switch it off.
In my experience, the editor that one obtains with Mathematica is very convenient and fast to type, if one remembers the hotkeys. Since he already uses Mathematica, I would like him to use this editor.
This is the reason to ask you if you have an idea of how to definitely switch the Check Spelling off?