There is no reason for an IP address to stay fixed for the entire session, though there may be a tendency to do so - but if a piece of equipment needs to be taken out/rebooted in ISP then connections may be dropped and re-connected.
if the police were seeking the location behind the IP address, then they would go to PlusNet and get the account at the time - PlusNet's systems record when the IP address was re-assigned, so unlikely you'd get a visit.
Discussion forums don't have access to that information and are unlikely be able to do much with it - where as banning an IP address is easy, but often futile as it is often easy to work around.
Assuming the person isn't on your connection, other than the odd pain dealing with such bans there isn't a huge amount to worry about - personally I got a fixed IP address from PlusNet which would generally stop such things, but I got it so I can remote access home
Nostalgia is memory with the pain removed