"If you find parts of the web un-usable, then don't use them."
The problem is that's closing the stable door after the horse has bolted. I don't know about the others, but my adblocker is there to protect my machine preemptively. I don't want to discover the site is abusive only after the damage is done.
I'd go along wholly with the idea of a warning system if it were a) reliable and b) somebody else's baby -- they won't be able to count on the likes of me to report abusive sites because I don't intend to be in the position of being abused.
Surely detecting such sites automatically should be a piece of cake.