You are probably fucked in that regard. Unless the miscreant happens to be in the same state as you, local cops aren't interested. And the FBI won't look at anything that doesn't have at least $10k in demonstrable damages. At least that's how it went when I tried to deal with a loon who was DOSing a friend's side project.

It should be easy to establish that a site that generates $200-$500 in ad revenue per day is worth more than $10K.

Under US law you would use the civil court system to recuperate losses/damage (crimes against a person), and criminal court system to place the perpetrator in jail (crimes against society). This really is a bleak abstract of the US court system, you really should talk with a lawyer if you are considering further legal remedies beyond what ICANN policies offer.