The first rule for preventing spam on the servers is to know who your customers are so you don't sign up any spammers who intend to abuse their accounts with you. We have a very strict anti-spam policy and recommend all resellers post a similar policy on their websites (see http://valuereseller.com/aup.html for our AUP).

We also run our own custom blacklists at the server level. If you find you are continually being spammed from a specific domain name or IP you can send a request in to support that we block that IP or hostname from the servers entirely.

The other thing you can do is install client side software to block unwanted email. Here are are a few you can try:

Spam Inspector

Surf Control

Concentrico Kill Spam