That's quite an easy change, and I just implemented it.

It will hide all threads started by users on your ignore list from the thread listing (and search results), but it will not prevent them from appearing in the "latest post" column on the forum index or on the site homepage. If you do by some change stumble into a blocked thread, you'll see a nice explanation message.
Making this configurable would be very difficult since it would require the addition of new database rows.
Enjoy!