Try not to make a point of constantly doing this. As far as I am aware, there is no governing body within the newsgroups. Deal with these people as best as you can and, my suggestion, don't let it bother you. Minimize the crossposting of messages. Crossposting is posting the same message in multiple newsgroups.
Some newsreaders allow the simultaneous posting to multiple newsgroups.
Make sure that when you do this that the subject would be related to the specific newsgroups and would be of interest to those participants in those newsgroups. Replies to posted requests or offers should best be done through e-mail to the originator of the message. Unless your reply would be of interest to everyone on the newsgroup. Another issue that is normally dealt with in FAQ's is the issue of quoting. When replying to a message make sure that you quote enough of the message that you are responding to so that others can follow your thread.