Another board I post on has an ignore feature, and it's not like hiding someone on Facebook, or having them disappear completely. If you had me on ignore, you would still see that I had posted in a thread, but instead of seeing the content of my post you would see a message that said "This user is on your ignore list. Click here if you would like to see this message, or here to remove the user from ignore." If someone else quoted my post, you would see it in their post, but not mine. If you have someone on ignore, it makes no difference to whether they can see your posts or threads, unless they also have you on ignore.
Also on that board, no one can ignore admins, and admins can't ignore anyone.