We used to have a lot of issues in the past. We have to take some drastic steps to prevent it because it impacts other clients. What we ended up doing was as follows. This may not be the solution for everyone, but for us it cleaned up the network and fast!
1. You make sure your AUP clearly details what you do, and do not allow.
2. We have developed a network monitor to check for outgoing mail. There are freebies out there as well.
3. We have developed a IP reputation monitor that kicks out a report twice daily.
4. MaxMind fraud checks for ALL new orders. If you do not pass it, no service.
It will let you know if any of your IP space (whether you own it, or getting from elsewhere and have it swipped) is racking up black listings. This is one of the easiest ways for us to nail em within 12-24 hours, and have proof to prevent chargeback/dispute issues.
You have to be mindful of both the abuse (catch quickly), and processing, issues (PP/MA).