Same problem here.
I'd like to have the spellchecker on for most note types (upcoming tasks, personal journal, recipes, etc). But I also take notes on programming tasks and computer games, both of which feature non-standard vocabulary that is sometimes not recognized by the spellchecker at all, so I can't add those words to the dictionary.
Being able to disable the spellchecker for just a note or, even better, a notebook, would probably be good enough. I do want the spellchecker turned on for those normal English notes, but being able to just turn it off for all notes under a given Notebook (eg "Gaming") would be fine.
My Skyrim notes are riddled with the word "dovahkiin" in a red underline.