We pay thousands of dollars annually to keep our IP#s and we like to keep them clean. The following works for us:
We set what some people might call a low daily email limit.
We've signed up for an AOL Feeback Loop (aol.com) so AOL will notify us each time one of their email users reports an email from our server as spam. This creates some false positives (which we simply ignore), but since most spam lists include lots of AOL addresses on it, this gives us very early warning of spam coming from our servers.
As soon as we've gotten an AOL email which shows our server is sending spam we change the Maximum daily email limit for that account to 1 (as 0 means unlimited) and notify the user (or user's reseller if applicable) about the problem and let them know they need to fix the problem before we'll allow them to send email again.