Strange. That should've disabled spellchecking unless I made a mistake with the code I gave you. Did you close JRNL before editing the Preferences file, or restart it afterwards?
For what it's worth, the original version of my file looked identical to yours. I deleted en-US from the dictionaries list, and JRNL added a few other spellcheck-disabling entries on its own when I next started it. You could try one last thing and replace the contents of "Preferences" with the following (make sure you've quit JRNL first):
{"spellcheck":{"dictionaries":[""],"dictionary":""}}
If that still doesn't work, there's a spellCheck attribute in JRNL\resources\app\summernote.js you could try setting to false. I'm hesitant to test it on my end for fear of possibly breaking something, but hey, might work for you.
Hopefully the author will come along and add a spellchecker toggle to the configuration menu, as well as fix that other entry-wiping bug I mentioned in another comment below.
Unfortunately, I don't really know of any alternative journal software, as I only started journaling again after discovering this app through the Ukraine bundle.