There are people on dialup who get dynamically assigned IP, and who travel and get somewhat random IP.
most ISP's have a domain range, which generates a max number of ip# The problem is if your block the whole range, you can be blocking an entire Sevice provider and all their customers.
The email registration account setup system is by far a much better way to manage accounts.
the person abusing it looses the account and no matter if they are using a ip generator, they would have to go through the trouble on creating a new email and new account each time, which is time consuming. Eventually they give up out of frustration.