once, we had a less-than-skilled attack on a company i was network admining at. I traced the source down to an ISP in a South American country and ISP and I contacted them stating that such-and-such IP on their network was engaging in an attack on my company. I asked them to look into this and block the user from hitting us thru the routes I provided. They said there was nothing they could do. I asked them what other recourse I had. They told me there was nothing I could do but shut down our systems and hope it went away. I asked them if I could take action to stop it and could I get and e-mail statement to that effect. They sent me an e-mail stating there is nothing they could do and I could do whatever I needed to correct the situation.
I ran it by the legal guys. got a thumbs up. put on a darker hat.
moved a bit of traffic off the oc-12 we had and proceeded to clobber the offending IP address and the nodes at the far end (ISP equipment). I got a very polite call after about an hour telling me that the offender has been pulled off-line and asking if I would be so kind as to stop defending myself as it was killing their network. I stopped my defense and was given a few names with contact info to call in the future should the needs arise.
a good result.