I don't think that can be done. I'm not used to ignoring or blocking people, and as of now, I don't have anyone on ignore. I do know this, however. If you ignore someone that's started a thread in one of the forums, I don't believe you'll be able to see that thread at all.
I guess that's a chance someone has to take. If the person ignoring discovers they can no longer see the thread, and the person liked that thread, then the obvious choice would be to take the person off of ignore that started the thread.