Depends on what you mean by "offensive", obviously.
Also depends on how you're reading these groups. If you read via usenet,
there's no filtering available (though you likely wouldn't complain
about what's seen via the web interface). If you read via the msnews
server, as with the web interface, there's considerable filtering done
automatically.
There's also some manual filtering done, obviously, as evidenced by
posts being removed after a time. Weekends seem to be the worst in terms
of offensive posts that get through, but the posts are generally gone
relatively shortly.
Nobody can stop all offensive posts without making this a moderated
group (which would dramatically increase response times). OTOH, in
general, they're pretty easy to ignore.
