In terms of "going after" the culprits I wouldn't bother unless the service attacked was mission critical, or there was a breach of security that resulted in confidential information being stolen (which if the information belonged to a 3rd party you should have already made an announcement) or if there are other tangible damages (i.e. reasonable grounds to claim for loss of business) beyond the costs of cleaning up the box.
Otherwise the CPS will probably decide (if no confidential information was stolen) that the costs of pursuing these people isn't worth it for what amounts to criminal damage. The box you were attacked from was in itself a compromised machine (if these people have any common sense) and probably that accessed from a hard to trace connection (Free wifi access etc). The aliases alone aren't enough to prosecute, even assuming you can resolve them to real people. So if you go the private prosecution route you are going to have to do a lot of expensive investigation to get anywhere.
At a technical level, take an image of the disk (or as MJR says, replace it) and rebuild the box from the ground up. You found one root kit but unless you can verify the integrity of every executable and library on that box (with off machine tools in case the tools themselves have been compromised) Then you really don't know what else might have been done.
Finally scrub any thoughts you have about counter attack, a. you will totally destroy any chances you have of legal recourse if the attacks continue and b. you will be as much the wrong side of the law as they are. Stick on some extra passive monitoring if you want just in case they do something really silly to reveal themselves. Maybe look at a decent IDS ?