gnus-crosspost-complaint

gnus-crosspost-complaint is a variable defined in `gnus-msg.el'.
Its value is

"Hi,\n\nYou posted the article below with the following Newsgroups header:\n\nNewsgroups: %s\n\nThe %s group, at least, was an inappropriate recipient\nof this message.  Please trim your Newsgroups header to exclude this\ngroup before posting in the future.\n\nThank you.\n\n"

Documentation:
Format string to be inserted when complaining about crossposts.
The first %s will be replaced by the Newsgroups header;
the second with the current group name.

You can customize this variable.