Hm. I looked around a little and found the following. Does it work?
If you later change your mind about ignoring a thread you can see the threads you are ignoring in your "My Account" area and clicking on "Ignored Threads"
I thought it might be somewhere in my settings, but I haven't been able to find it. I don't appear to have a "My Account" section. Is that for community supporters only, by any chance?
Yeah, I did a google search and found an older thread that said you couldn't unignore threads, but it was from a year or so ago, so I thought maybe a solution might have been introduced since then. It's weird that you'd be able to ignore a thread without being able to undo it.