The idea is that you don't use the IP address to ban posters but to report them to their ISP. The ISP has logs of who is using each IP address at a given time so it's trivial to match the address to an account. ISPs take that sort of thing seriously, and it usually results in an immediate termination of service.
It's true that people can use an anonymizing proxy to post without revealing who their ISP is, but then you block the proxy's address. There's no legitimate reason for using an anonymizing proxy for accessing this site so that's not a problem.
I've worked as an administrator. Admins like an easy life and complaints interfere with that. In addition, many admins remember the days when everyone on the Internet behaved responsibly, so they tend to come down like a ton of bricks on anyone under their jurisdiction who misbehaves. (In particular, they detest cocky little brats who think that posting from home means they can get away with the sort of conduct that would get them beaten to a pulp if they tried it in real life, and most admins will stop at nothing to put these people in their place.)
Perhaps it might be possible to prevent embedded images for unregistered users.
Incidentally, posting dummy messages in other threads to knock the offending one off the screen doesn't work. I'm sure many people use the "view new posts" link to see all threads that have new messages, and everyone who does will not only have seen the offending one but will also have had the additional irritation of wading through dozens of pointless extra posts.
What we need is moderators who can take rapid action at any time.