If the spammer spoofs a user name using UNICODE characters, AVH creates a "Report Spammer" link in the notification email that produces a 406 webserver error when used.
The parameter portion of the report link ends up looking like this:
?action=emailreportspammer&a=%EE%ED%EB%E0%E9%ED&
The %EE%ED%EB%E0%E9%ED is what messes up the Report Spammer link.
http://wordpress.org/extend/plugins/avh-first-defense-against-spam/