How To Remove Scab On The Back Of Head And Bottom Of Skull?

I have had a scab on the back of my head at the bottom of my skull for over six months. I pick it..it bleeds...but it won't go away. I put peroxide on it. It also seems to have a crater in the middle of it when I pick at it. I got spider bitten..multiple bites, and they healed, but this one did not.