If you want to lower the score of crossposts, the line to match on is the Xref header.

("xref" (" talk.politics.misc:" -1000))

If you want to lower the score of articles that have been crossposted to more than, say, 3 groups:

("xref" ("[^:\n]+:[0-9]+ +[^:\n]+:[0-9]+ +[^:\n]+:[0-9]+" -1000 nil r))

This is generally not a very good idea---it takes a very long time. Gnus actually has to fetch each individual article from the server. But you might want to anyway, I guess. Even though there are three match keys (Head, Body and All), you should choose one and stick with it in each score file. If you use any two, each article will be fetched twice. If you want to match a bit on the Head and a bit on the Body, just use All for all the matches.

You will probably want to mark articles that has a score below a certain number as read. This is most easily achieved by putting the following in your `all.SCORE' file:

((mark -100))

You may also consider doing something similar with expunge.

If you say stuff like [^abcd]*, you may get unexpected results. That will match newlines, which might lead to, well, The Unknown. Say [^abcd\n]* instead.