Yes, some of them would create 1 or 2 new accounts, but I don't think they will try it much more times if they get always banned after some time. This is a risk I would accept.
Blocking IP ranges like 80.1.2.* is always annoying for other people in this range, especially if they are new to the game and don't know why they are banned. Also another message would be helpful. Something like this "Your IP or IP range has been banned. Please try it again in some days."
But if you use IP ranges, they should always be as small as possible. => Never use hostranges like p54B9%.dip.t-dialin.net. % is a wildcard for 4 characters, including digits and letters. Thus you block 36^4 = 1.679.616 possible IPs. This range is even bigger than a 80.1.*.* range, which includes only 256^2 = 65.536 IPs and is used very very rarely.
