Actually, what you're wanting (them not showing up in your Inbox) is on the to-do list. But not in the way you're talking about.
I'm never going to make it so that putting someone on ignore means they can't reply to one of your posts. It's one thing for a moderator to ban a user from a premium board, but quite another to let anyone use the Ignore button to interfere with another person's posting ability.
What I'll be doing is adding a routine to post-submission so that if they're writing it to someone who has them on their ignore list, it'll simply mark the message as having been read so it won't show up in their inbox.
If not for how horribly I wrote the Preview/Spellcheck routine, I could implement this in about 30 seconds. As it is, it has to wait until I rewrite the whole Post/Preview/Edit/Submit/Update set of routines to work the way they do on SI.