I wrote a response to this earlier, before the company's wireless network had a brainfart, so I'll re-write it even though Admin has replied.
IP addresses are handed out by ISPs, so all anyone needs to do to get a new one is re-boot their router. Once that is done, the old IP address will be allocated to someone else, and that person will be banned by association.
On top of that, you can find a gazillion proxy servers out there, which will present as a different IP address anyway. Very useful for people in the US who want to watch BBC videos, crap for admins to limit access.
There are also, as PpPete says, other ways of masking your IP address easily available.
In summary, the people who are most frequently caught by IP bans are the innocent ones so they are, IMO, worse than useless.