if someone is sending you http requests with ficticious http headers then there's nothign at all you can do about it, as they are making them up themselves... you could in theory match maybe for known strings within iptables, but often host headers are missing anyway. personally i'd just say you should suffer in silence...
if it's more the noise in the log files you dislike then each virtual host can use their own log file, so you'd have a virtual host for your real site and then let any hits against the default site just be logged to /dev/null or such like.
http://httpd.apache.org/docs/1.3/logs.html#virtualhosts
