I've been looking, and was reminded of noting this when HSX was revamped, as we waved Goodbye to the original Killfile.
When you look at a main forum page, all of the forum topics (outside messages) are listed with the author's current handle, if there is one. If you click a topic to view the inside message, you see both the user's handle and the registered username.
On the old HSX, every topic and inside message contained the registered username. You could put a username on the killfile list, then, with each forum page load, filter out any messages associated with that username.
With new HSX, I'd have to put the user's handle in the killfile. That would work, until the miscreant changed their handle. Imagine that! It would incentivize a cat and mouse killfile game, and chaos would ensue.
OK, I could try to automate the mouse catching: When a handle is added to the kill list, I could open the inside message link in the background, and also add the registered username. But, that would only be marginally effective. If a handle was changed, the user's topics would be visible, until you clicked on an inside message, at which time the new handle would be added to the kill list.
There's one more way the screenings could be done, but, that would involve opening every link on the page to get the username for every topic... ugh. Obviously, that's a non-starter.
So, there you have it... (sorry grammar)