ok i see but i see he also puts letters M I 5.
you can use this filter:
"M{^a-z}I{^a-z}5"
i see he cross-posts as well in this particular case, but anyway you can delete headers on entry with this kill filter, so also if he won't cross-post it will work.
so how you do it:
edit menu->filter editor
then press green arrow button to insert new entry.
name e.g. "MI5 kill filter" you may leave only "kill filters" checked if you like.
then enter filter
subject contains "M{^a-z}I{^a-z}5"
or (it appears this signature is always in the beginning of the subject).
subject begins with "M{^a-z}I{^a-z}5"
(double quotes narrow the matches further)
press ok.
then in edit menu->properties->kill filters you'll see "MI5 kill filter" under "available kill filters".
select the topmost "Newsgroups" entry in the middle pane, select "MI5 kill filter" in the first pane and press the "Add" button, you'll see it appearing under "Kill filters" rightmost pane.
then press OK.
then newly incoming headers of the spammer will enter as deleted, e.g. if you reset the newsgroup and redownload headers then check "del" in the quick filter dialog you can see all those spam posts as deleted.
i checked with search service it appears only the spammer posts are hit, it found 3279 such posts in the indexed (binary) groups, only one other post which contains "M.I.5" in the middle.
i see (binary groups posts) he doesn't post regularly looks like it started two weeks days ago, maybe some kind of periodic mental disorder, full moon, new year i don't know