I`m with HandsOnHosting on this one - having initial fraud checks during order is a must. MaxMind is quite flexible in this aspect and the initial Phone Verification clears the clouds. I can also add that for some cases a Phone+ID verification stops the spammers in their tracks. Mind you, hosting companies are evolving but the spammers, hackers, etc. are evolving too. Constant monitoring is the key!
Of course having a limit for the outgoing email quota is also helpful, you can also provide spam filters like SpamAssassin available for customers so they can also protect themselves from unsolicited emailing.