unfortunately, mi5 has now switched over to altopia, and they aren't
known to stop this sort of thing

-we can try to report him, but their tos/aup gives extremely wide
latitude as to 'acceptable' posting behavior.

https://www.altopia.com/detailedterms.html

"Altopia has no obligation to monitor or remove information on the
System and Customer acknowledges that postings and communications by
other users are not reviewed, monitored, screened, approved or endorsed
by Altopia."

"#

Content. Customer shall not use the System to post or transmit any
illegal material, including without limitation any transmissions that
would constitute a criminal offense, give rise to civil liability, or
otherwise violate any local, state, national or international law or
regulation, including without limitation United States export control
laws and regulations."

one can try under:

"Q: What is your policy on cross-posting?

A: Cross-posting ability is not to be used by customers for the purpose
of "cascading". We may disable the cross-posting ability of a customer
if we believe they are violating this policy."

but -imo- the volume of mi5 posts does not rise to the level that
[below] can be used:

"Q: What is your policy on message floods?

A: If a message flood is resulting in a denial of service, we will stop
it. If a user on Altopia is causing it, we may disable their posting
ability. If an incoming flood is making a group or groups available on
Altopia be unusable, we may delete the message flood or prevent it, so
as to make the group(s) be usable. A denial of service is when news
servers are unable to perform their intended function because of the
overload caused by a message flood or other harmful activity. Readers
inconvenienced by numerous off-topic or inappropriate postings does not
constitute a flood or denial of service."

you could try under this clause:

"Q: What is your policy on non-advertising excessive multi-postings
(EMP)?

A: We consider excessive multi-postings to be a flood and we react to it
as described above."

long shot for this part:

"Q: What is your policy on off-topic postings?

A: We do not have a policy against off-topic postings and we rely on
users to operate in a way that they feel is appropriate for Usenet
and/or the groups they are participating in."

likewise, this would be a long shot:

"Q: What is your policy on a user posting with a bogus return address?

A: We feel users have a right to decide whether to be anonymous or not
and so we do not have a problem with a user posting with a bogus return
address. The news server software does require that a return address be
in Internet standard format, such as foo@bar, but the address does not
have to be real. We do require that users verify that the address they
choose to use is not actually someone else's valid address or domain.
For example, bar@foo.com is actually a valid email address, because
foo.com is a registered domain."

all in all, things will now be a bit more diff. for those who
don't/can't filter the poor soul