Trying to ban spam by IP address is a poor idea. First of all you won't know what IPs to try to block until after you've got spam, so it would only help with returning spammers. That's complicated by spammers using continually changing IP addresses and many of them using bot nets of compromised computers so the IP addresses are in block that may contain legitimate users. Since most people get IP addresses assigned dynamically, receiving a new one every time they connect, the more IP addresses you block the greater the likelihood of blocking someone who shouldn't be.
Early on I added IP blocks of places that none of my users would be from, the former Soviet Block, parts of Asia and Africa as an attempt to cut down on spam. Once I managed to stop it all automatically I didn;t need the IP blocks, but just left them. That was until about a week ago when a user emailed me about being banned from the forum. Turned out their ISP got bought out and they were now getting assigned an IP address from a block in Africa. So you never know who you'll be affecting, and if they don't tell you and instead simply go elsewhere, you'll won't know you lost someone. Needless to say I've removed all the IP blocks I had.