It looks like http://www.mail-archive.com/fossil-users@lists.fossil-scm.org exists (although mail-archive.com seems to be down at the moment, so not sure that's the exact correct URL), so perhaps just suggest that users go there on the http://lists.fossil-scm.org:8080/cgi-bin/mailman/listinfo/fossil-users page.

anonymous claiming to be dclark@pobox.com added on 2009-04-13 02:55:49:
Looks like this was already reported via email: http://www.mail-archive.com/fossil-users@lists.fossil-scm.org/msg00079.html

Not sure if the stated solution didn't work or something, but the *really* trivial work-around would just be to put both the correct pipermail URL and the mail-archive URL in the list description section, along with text saying to ignore the archive link below.