With proper order screening and scripts setup to monitor SMTP connections we've cut down on customers signing up to send spam in the first place. However of course nothing is 100%, customers websites do get exploited, ect. In the event that we receive a notice in regards to spam, we manually investigate it. If found that it's valid and the customer is indeed sending spam the site/VPS is suspended and notice is sent to the customer to contact us when they're online so that they can resolve the issue.
It's also important to note that some spammers will claim that their VPS was hacked (even though it wasn't), request a new OS install, and then start sending spam again.