The format of addresses on IRC is the following
So hostname is just the part after the @. It's more clearly displayed in a /whois of a user. This always changes if it's a dialup user each time they reconnect to the Internet, so a combination of wildcards may be needed to ban the user. Inevitably though you could end up banning perfectly innocent users from the same ISP. That's one of the reasons a ban evader can be a pain in the ass though and there's not much to be done about it
Sometimes a hostname does not resolve and an IP address will be displayed instead. You can also obtain the IP through the /dns command (or obtain the hostname if the IP is displayed).
The 'user' part is aka the IdentD or UserID which can be changed almost as easily as a nickname, it's just "newbies" know how to do it less than more experienced users, plus it requires a reconnect.