Poor you. The responses to your post are in the majority rather unhelpful, but I suppose that's because a phobia is an irrational fear, and it's difficult for someone without phobias to relate to your irrational fear of spiders.
First of all, you need to tell yourself that if you keep pinching and rubbing the skin between your thumb and index finger, it is going to tear, and it's not exactly an easy place to stick on a plaster. Keep trying to 'clean' this piece of your hand and you're going to actually hurt yourself. Sometimes dealing with a phobia is easier if you try to force yourself to look at the facts.
Shaking off an association with something you hate is really difficult - could you perhaps get a close friend or member of family to hold that part of your hand for say, ten minutes? In that length of time you won't be able to scratch the skin as you'd like, and you'll begin to associate the person you like with the skin, rather than the spider.
I hope you feel better soon.