I Think you are slightly confused with your acronyms. ISP stands for Internet Service Provider. I think you mean the IP address which uniquely identifies a node on the internet.
My understanding of the problem is that although broadband users are allocated a permanent IP address, dial-up users are allocated one dynamically every time they log on. I use dial-up (yes, I'm an old fuddy-duddy) and my IP address changes every two hours because my ISP limits access to a maximum of 2hrs at a time. That means that a username is not associated uniquely with any particular IP address. It also means that different users (with the same Internet Service Provider) may be allocated the same IP address at different times. Members cannot be barred purely on the basis that they are using the same IP address because they could be two genuinely different legitimate members on dial-up. (unlikely - but possible).