There is a fine line between annoying and intolerable. Personally, I have danced across this line so often that I am very familiar with the difference. Unfortunately, there are some characters who are completely unaware that there even is a line.
All readers have met unlikeable characters sometime in their book adventures. These are the characters who make your blood boil and your veins bulge in frustration. For some reason or another, they get on your nerves. In fact, they are a common literary tool used to stir up emotion within the reader. Think about all of the romances that are hate-to-love. In the beginning, we all hated Rhys from A Court of Thorns and Roses (and even though some of us still hate him, you get the point), Will Herondale from The Infernal Devices, the Darkling from The Grisha series, even Haymitch from The Hunger Games. When I think about it, there are a lot of characters who started pretty high on my "I'd-love-to-watch-you-burn" list, but ended up on my list of favorite characters. I classify these characters as "unlikeable" since they rub me the wrong way sometimes, but have not completely lost their chance for my respect.
But, there is another kind of annoying character. The kind that crosses the line between decent indecency and hateful scumbag. These characters are definitely unbearable, but sometimes it can be hard to discern when they changed from merely unlikeable to full-blown intolerable.