I'm not sure what the reason that this isn't implemented is, but since there doesn't appear to be a route to update the ignored list in the same way as the star updates the favorites list, it's not as simple as just dropping in the link.
It still seemed like people might find it useful to have though, so since you hadn't gotten too much feedback on this yet I went ahead and wrote a userscript that adds this functionality. In addition to the favorite star, there's now an ignore radioactivity symbol*:
Once I'm more confident that it works reliably (and I successfully avoid getting scolded for scraping the user preferences tab) I'll write up a post for it over on StackApps. In the meantime, if you want to test it out, here's the information:
install / update script – view source
*I'm open to suggestions on this one. I wanted a "No Entry" unicode symbol originally, but the ones that I found didn't seem to be well-supported.