One thing you could do to provide evidence is do a demo(a recording) of them doing whatever you said you want them banned for.
In console you can type
- record [demoFileName - preferably in the format of MM-DD-ServerName-ObjectName-HH] to start recording the demo.
- stop - to stop recording the demo.
- demoui to get a nice little interface for playing back the demo.
please upload the demos onto a free file hosting service(preferably not rapidshare Q.Q) ex: MegaUpload, SpeedyShare, and even some others.
If you want, you could even record a video of the demo after you record the demo itself and post that to youtube if you'd prefer that instead.