If they are online, their name will appear in the list of "players in chat" and it will be crossed out ([del]Player Name[/del]). You can click on their name here and an option "don't ignore anymore" will appear. You can click on this option to cancel it.
Agreed, confirmed by personal experience. Sometimes you just forget why you ignored them because you haven't seen their talk for some time and when you unignore them and you see them talk again, you go like "Right! That's why I did it."