Well, here's an idea. Give sysops emergency power to ban a logged-in user (by name), while simultaneously blindly banning their IP address -- temporarily. And they'd be honor bound to report the ban to the list, just as when a sysop protects a page to stop an edit war.
For example, user:Skeezix messes up a lot of pages, so sysop BigCheese spends countless hours cleaning up after him and finally says it's not worth it. After some discussion on the mailing list -- or in a really urgent case, unilaterally -- the sysop presses the magic "Ban this logged-in user" button.
Whereupon two things happen:
1. The user's account is blocked.
2. The user's IP address is blocked.
And one thing doesn't happen:
3. No user, not even a sysop, can see the blocked IP.
So if Skeezix tries to log in as user:Spreitel -- he can't because his IP is blocked.
Advantages:
* Stops the vandal cold.
* Avoids revealing IP addresses.
Disadvantages:
* Like all banning mechanisms, it could be abused.
* If the IP is shared by "innocent" parties, they also get blocked.
Ed Poor