Technically it's possible for anyone to get the ip address one way or another, though wouldn't do much for non-mods/admins (unless you have open ports).
I'm assuming that site administrators held the power for themselves because the rule to not post when your baned is held rather loosely. The only time that serious action is takened is usually when a user who was banned comes back as a alt to spam more. This isn't to say to make a new account now and spam/just use the other account, as you never know when a admin is on.
Furthermore, ip banning is rather redundant as the unfortunate fact that it's fairly easy to spoof ips and use ip masks/proxies or even to just change your ip. No matter what moderators do, to change our ip is so easy you can't stay banned forever if you know how.
As of last thing to point out, it's rather childish and a waste of your time to go through all the trouble to take 5 minutes to post more spam than for the 5 seconds the mods can 30-day ban you hm?