That is exactly the middle-management mentality that causes so much trouble: "Look, it works, it killed the fly. Oh, and it also killed the whole village..."
Maybe you would be better off plugging the network cable off - voila, stops spam 100%. The problem is that blacklists like these create a lot of false positives - legitimate customers of other ISPs whose IPs and IP ranges got blacklisted can't mail to your clients. Not your problem? Wrong, it is your problem, because by applying the blacklist "solution" without proper consideration, you are punishing innocent people collectively (bad practice).
Imagine an ISP with outgoing mailserver for many ADSL users - many users today have compromised Windows instalations serving as zombies for spammers. ISPs cannot effectively prevent this, yet when the zombie awakens and spams, everyone else using the same server is punished by the blacklist.
You are just moving the hassle to the other guy, which is once again, bad. There is no "magic bullet" solution to spam, but it pays to be considerate and responsible. And if your customers leave because they can't receive legitimate mail, don't be surprised. And if your answer is "It ain't my problem", you don't deserve them anyway.