If you can gather enough "proof" about the attackers you certainly can go to thier ISPs with a complaint. I've actually gone to to several "suiciders" ISPs with a DoS. Most of the ISP responded quickly, pulled the persons connection for a day or so, and then put them on "probation" after they signed a "promise to be good note".
For details on how to do this, see the "How to Deal with Suiciders and Lamers" article in the UT tutorials on
www.unrealadmin.com. (Full disclosure here... I'm the author 8) )
GL HF
|LCN|DeadMeat
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I have a dream!
Where all UT players: red; blue; gold; and green, all play together fairly and without cheats!