Be very WORRIED!
I'm really surprized that you haven't received complaints or found that your IP(s) have been blacklisted already.
You must be one of the few web hosts in the world who knows one of their customers is massively SPAMMING, thereby putting ALL your other clients at risk, as well as your own operation and letting them continue doing it...
Seriously, ditch this client!
David
Free AdSpecs Link Exchange
http://www.adspecs.co.nz/links/