Not easy or it would have been stopped by now. If I try to register an IP, email address and user name is recorded.
If I reconnect to the Internet (dial-up) the IP is dynamically assigned, meaning it is different, so if I register again and use a different email address and user name, no problem all are different.
We have to make it harder to block the spammers but not to the detriment of legitimate users needing to register, so it isn't as easy as you say. Dynamically assigned IP get used again by other users on that ISP, if they happened to try and register using an IP already blocked, an innocent user gets tarred by the same brush as the previous spammer.