Welcome to Usenet!

"Usenet (USEr NETwork) is a global, decentralized, distributed Internet
discussion system that evolved from a general purpose UUCP architecture
of the same name. It was conceived by Duke University graduate students
Tom Truscott and Jim Ellis in 1979. Users read and post e-mail-like
messages (called "articles" or "posts") to one or more of a number of
categories, called newsgroups. Usenet resembles bulletin board systems
(BBS) in most respects.

One crucial difference from a BBS is that there is no central server,
nor central system owner. Usenet is distributed among a large,
constantly changing conglomeration of servers which store and forward
messages to one another. These servers are loosely connected in a
variable mesh. Individual users usually read from and post messages to a
local server operated by their ISP, university, employer, or some other
local organization. Then, the servers exchange the messages between one
another. Usenet has been described as a system of online collaboration
and interaction similar to today's Web 2.0.[1] It has also been pointed
out that its decentralized structure makes it more democratic than Web
2.0.[2]"

http://en.wikipedia.org/wiki/Usenet

No, there is really no way to get rid of off topic posts.  If you can
identify the ISP of a spammer you  can complain to them and sometimes
get their account pulled but mostly about all you can do is filter them
out or ignore them.

Jeff DeWitt