Lets say you guys had a member here (someone you knew in real life and could trust) that was a regular and used the forum in a normal fashion and did so from a static IP address (its always the same).
Suddenly, you see someone abusing the place and the connection is the same IP address as your friend. Alarm bells.
You would know it's not really them so the question is:
Since their IP address is static, could someone be tapped into their IP address?
Could someone be spoofing their IP address?
Since you know it isn't you friend doing it how could you detect whether it is #1 or #2.
Most people that "spoof" an IP are usually trying to hide their own IP, not emulate another specific IP. True IP spoofing can be accomplished but it would simply be the IP transmitted in the packets and is still generally done through a proxy. The problem is that the spoofed IP will collide on the internet with the true IP. If the true IP is static and on all the time, it makes it even more difficult.
The more likely scenario would be that someone has found a way into the system of the IP subscriber, probably through a wireless connection.
If the person did get in, the odds of that person randomly connecting to the same website that the legitimate IP holder visits are astronomical. If he did, it would almost certainly be someone he knows and has an issue with.