There are a few pathetic individuals in Usenet newsgroups so completely without dignity that any interaction is a positive experience. Unlike your basic NG idiot these parasites cannot be chased away.
If provoked, flame wars will consume the group providing the kook the attention that he so desperately craves. This will turn the NG into a sea of negativity chasing away many good people & threads.
The proper response to one so identified is to ignore the kook COMPLETELY. Put the kook into your killfile. Steer the topic away from any responses the kook may make.
Some otherwise decent contributors cannot or will not ignore the kooks. Several group Old-timers should politely suggest in private email or publicly this policy of disengagement. If the contributors cannot be made to understand that they are adding to the groups malaise they must also be killfiled.
This policy has been shown to work in established newsgroups that were "taken over" by kooks for years.
Also be aware of bands of idiots that systematically target a newsgroup for a "takeover". These morons pose as individuals that have not met before and support any outlandish garbage the other may espouse.
It is also worth the time to complain to the person's ISP. A polite email sent by many members of the group to admin@[kooksISP].com, root@[kooksISP].com, and webmaster@[kooksISP].com has also proven to work wonders.