Fraud and abuse prevention really boils down to something like this. You don't need to be perfect with blocking these folks, but you need to be better than you

Fraud and abuse prevention really boils down to something like this. You don't need to be perfect with blocking these folks, but you need to be better than your competitors so that the cost/benefit balance shifts to encourage the user to go elsewhere. Unfortunately trolls don't disappear, they just go somewhere else. Similar to the saying about the hungry lion. If a hungry lion approaches your group while hiking, you don't need to be faster than the lion, you just have to be faster than at least 1 of the other people in your group.