From another newsgroup:

The MI5 poster:
His name is Mike Corley, he is an IT professional but alas he has
mental health issues. Here are some details about Mike Corley.
He lives in London, UK. He first began his postings to usenet, BBS,
and forums in 1995, unsurprisingly first to alt.conspiracy.

He has his own website:
http://www.five.org.uk/

"There have been numerous newspaper articles and online thesis
about Mike's postings, this is a BBC article on the subject:"
http://www.bbc.co.uk/dna/h2g2/alabaster/A164404

"And amongst many other covers of the Mike Corley phenomenon
there has been a modern opera about this usenet legend:"
http://www.thecorleyconspiracy.com/

The problem is amplified by posters who reply to Mikes X-post floods
without snipping, and without removing the crossposts.
But also Mike Corley sets "follow-up" groups to his MI5 posts, this is
where he hasn't actually posted to a specific group but it's listed as
"follow-up" in the crossposts, the lusers who reply, their posts show
up in groups that Mike wants flooded.
Mike Corley has set uk.misc as follow-up to all his posts, (which I've
removed) it gets flooded with the replies, and of course the
X-posting. Mike Corley has been doing this since 1995, and he knows
what he is doing, he avoids killfiles/plonk/block by morphing and
using pay-as-you-go dial-up. Filters rarely have an effect as in this
example, if you set the criteria as "MI_5", then the flood morph to
"MIfive" or whatever. He dosen't post
via google dispite the headers, nor are the email addies valid, he
dances between proxy servers such as altopia etc, filtering has no
effect.
The simple rule with Mike Corley floods is to ignore and not to reply.
Mike will take his medication and eventually stop.....until the next
time. But if lusers do the foolish and reply/x-post/fail-to-snip,
then Mike Corley sticks around. So don't spam your pointless replies
across dozens of groups.