Depending on your mail software (mailenable for instance) you can limit the amount of emails that customers can email to. I limit customers to about 100 messages (or 100 email addresses) an hour. This stops them from sending out mailing lists through their email account. We offer a 'mailing list' program if they want to send out more than 100
Also the mail software might have a filter on it allowing any 'attachments' to be checked for virus.
I haven't had this happen myself, if it does:
I politely tell the customer that this is happening.
Tell them to install an Anti-virus program immediately
Immediately shut off their email as the customer doesn't want to get 'blacklisted' by other service providers.
In the long run you are helping your clients by making sure something worse doesn't happen like their domain name getting 'blacklisted'