I don't use it. Many years ago I had to use it for one person on another forum who loved to post disgusting pictures of himself; but everywhere else I just scroll down and past.
Ignore lists may seem like a blessing but they can also allow the pest to speak ill of the frienemy with impunity; furthermore, they can make for awkward situations in which the ignoree is unaware the ignorer is ignoring him and can acknowledge and/or react to comments the ignorer makes. This happened to me once in an IRC chat channel in which I had set one member to ignore: one night he got into a convo with a third member and I was replying to the last named, because I could see his messages, but not the other guy's; to make matters worse, the third guy didn't know I was ignoring the first one and assumed I was addressing both of them with my replies. Based on how I was given to banter it's quite likely the first guy became even more incensed at me as he assumed some of by jocular comments were aimed at him, but they really weren't—how could they since I wasn't aware of anything he was typing?
I say, if you're gonna ignore someone, at least have the courtesy to notify him, and the awkwardness will be avoided.
As for the first case, well, I did an experiment in that same channel years before the episode told above. Two of the guys were being jerks and I told them I would set them both to IGNORE mode presently; then started acting as if I had. Sure enough, they immediately started talking crap about me. At one point one of them said, "he's ignoring us, so", and then I promptly typed "uh-huh, keep telling yourself that". They said "lol" and piped down afterwards.
So, I'll just scroll down.