Well, I've found the way to achieve the goal without actually unbinding any events. I needed to replace the form action attribute with '#', the same with special submit link href ('#'), and... DONE! After my AJAX call completes (or fails), it restores original hrefs and submits the naughty form.
So the question I asked was wrong - my fault. I needed to prevent a form from being submited and this is way easier than removing alien events!
TL;DR - kill hrefs first, then restore them :)